Narrow issue
The reason why that particular question is overruled by wider SE meta discussions is because even if we (on SFF:SE) decided that we wanted to be able to link answers as duplicates rather than questions, a decision has already been taken on a site-wide basis not to allow that.
Since it would require a programming change and not just a policy change, this means that all the wishing in the world ain't gonna make it happen.
Obviously the flipside is that if we do decide (for some reason) to accept this as site-wide policy on the say-so of a single poorly upvoted answersay-so of a single poorly upvoted answer by someone who's not a member of SE staff, then we'd suddenly have to reopen potentially thousands of closed questions.
Wider issue
Individual sites are allowed to make their own policy through a process of concensus and opinion. It is a long-standing and reasonably popular policy on SFF:SE (and site-widesite-wide) that where an existing answer covers a newer question, the new question is marked as a duplicate in order to prevent multiple semi-identical answers being repeatedly posted, either by the original author (wasting their time) or by a new author (leading to accusations of plagiarismaccusations of plagiarism).
We're probably a little more aggressive about de-duping on the basis of answers, but that's largely because we have such as well informed core of users that dupe questions are picked up more easily, even when they're less obvious (at first glance) to less experienced users.