Does it make the answer better?
Yes it definitely does. Another person with <2k rep had suggested the same edit as you, and I had rejected it on the grounds that it actively harms the answer
Was Matt Gutting right in adding those parts?
It's not up to us. If it's their decision to add those parts to an otherwise (not my words) low quality answer, instead of posting an answer of their own - it's their decision.
Do we have the right to roll the edit back because the answerer "doesn't deserve the upvotes"? No! This site is not about upvotes and reputation - it is about building a complete collection of questions and answers. Matt Gutting's revision made said collection better, so there is no point in rolling back the edits because of the feeling of justice.
I had done similar things myself (e.g. here) with an otherwise low-quality post, and I had added information to other people's answers without asking first. the way I see it - when I edit, I offer my help with source search. If the OP decides to roll it back - then it's their decision, but at least I took a stab at making the post better.
If Daniel Roseman decides to roll back the edit - that's their decision, since it's their answer. Then the edits fall under "conflicts with OP's intent", and then it's not up to us.