The question should be reopened. Nothing in the question indicates that opinions are being sought. Of course, anyone could always post an opinion as an answer, but anyone could do that to any question. If we're afraid that for some reason people will post their unsolicited opinions, there are ways to deal with that. When people post unsupported opinions, downvote them. They will quickly get the message that this site is not looking for opinions to be posted as answers. If there is a particular problem with new users posting opinions, questions can be protected to prevent them from being able to post. There is no need to go around closing questions because people might post opinion answers.
That does not mean that the Primarily Opinion-Based close reason is useless. Some questions do ask for opinions, either explicitly or implicitly. Explicitly, by asking "What's your opinion?" or "Who's your favorite...?" or "How would you write the ending?" or other similar types of questions. Implicitly, by asking questions that do not contain enough information to have an objective answer, such as "Who was the best teacher?" (we don't know what definition of "best" is being used) or "Was Dumbledore moral?" (we don't know whose definition of moral to use).
Generally speaking, if a question can be answered objectively then it is not Primarily-Opinion-Based. To clarify something that was discussed in comments to another answer, a question is not not Primarily Opinion-Based because you could post a "we don't know answer". A question is not Primarily Opinion-Based because it can be answered objectively without opinions. It just happens to be that sometimes the objective answer is that there is not enough information in the existing material.
The above being the case, when we see a question that already has an answer based on objective material we already know that the question can be answered objectively (unless the existing answer wildly ignored the actual question).
It is important to remember that just because an answer is not an opinion that doesn't mean that the answer is correct. And on the flipside, just because an answer is incorrect that doesn't mean that it's an opinion. We have hundreds of questions that are not closed as Primarily Opinion-Based that have conflicting answers. It is the responsibility of voters to use their upvotes and downvotes to try to determine which answers are right/wrong, good/bad, or better/worse.
Now let's look at this particular case. The question author began by trying to prove that there were at least five floors in The Burrow. The author then continues and says "This being the case" before asking the question. That indicates that the question is predicated on the premise that there are at least five floors in The Burrow. If we didn't know how many floors there were then there would be no question.
The actual question then asks why there are so few rooms. Given that the premise of the question was that there are at least five floors, the question is assuming that five floors should reasonably contain more than five rooms. This seems like a fair assumption – as far as I know most normal houses fit more than one room per floor.
So now we have to decide whether this is Primarily Opinion-Based. Well, did the question explicitly ask for opinions? No. Did it implicitly ask for opinions? No. Is it possible that someone might post an opinion? Absolutely. But that possibility exists for every question, and we don't close them all. Can the question be answered objectively? Certainly. In order to answer the question objectively, one need only search through the existing Harry Potter material for an explanation as to why there are so few rooms given the amount of floors.
This explanation could theoretically be any number of things. There might be some source that shows that the Weasleys liked being squeezed together so they deliberately only had a limited number of bedrooms. There might be some source showing that that's how the house was built and it's impossible to change it. There might be some source saying that there's a maximum number of bedrooms a house can have.
The key here is to find the relevant sources. They may be scattered throughout the series, they may be buried in external material, or there may not be any sources that address the issue. If someone does find relevant sources then a good answer can be posted. If someone can't find relevant sources, the question doesn't magically become "what's your opinion?" The question remains "what is the factual reality within Harry Potter?” and the answer might be that there is not enough information to determine that factual reality. That is also not an opinion.
Of course, if someone is going to post an answer arguing that there is not enough information, a simple "we don't know" is not very helpful. One can explain at length why we don't know, or how we know that we don't know, and one can perhaps focus on certain details that we might know.
Now if we look at the existing answer we see that it is based on a collection of sources throughout the book series. The answer attempts to address the question by undermining the premise that five floors could and should contain more than five rooms. The answer does this by trying to show from the books that the floors of the Burrow were decidedly not like the floors of a regular house. Instead the floors were simply individual rooms that were added on to the house over time. If that is true, we can see that there is not in fact additional floor space that could have been used for more bedrooms, so the question falls away.
The answer doesn't pretend to answer all possible questions about The Burrow. One could still ask why they didn't add on even more rooms as additional floors like they did with the current rooms. But that would be a different question, and it would have a different answer.
Now, it is entirely possible that the explanation given in the answer is not the correct explanation. To that end, voters should carefully analyze it to see if it is compelling, and upvote/downvote accordingly. If it is found to not be the correct explanation then someone else can find a different explanation. But that has nothing to do with whether the question is Primarily Opinion-Based.
So in sum, the question is not Primarily Opinion-Based because it's not Primarily Opinion-Based. The existing answer merely confirms what we could already have determined on our own – that the question can be addressed based on facts from the books (and perhaps other material). By closing a question such as this one, we potentially prevent people from getting good answers to their questions.
The question should therefore be reopened since the current close reason does not apply to it.