I agree that in general it is preferable to ask a new question rather than to do, months latter, a massive overhaul of some question that is not ones own.
There are exceptions, however, and I feel there is an important detail missing in the description of the specific case. Namely, the question has two (good) answers, one of them accepted.
The point of the edit was (at least so my understanding) to preserve the existing answers. This is good practice.
In case of a question asked by an actively maintained account I would still recommend to check back with OP before making such an edit.
But the account that had asked the question was not seen since a long time, and has hardly any activity. What OP intended is not really all that relevant at this point. They accepted an answer, so it seems they got what they wanted, and left.
Our criterion now should be if the combination of the new Q with the existing A is a better and meaningful contribution to the site than what we have. If yes (and it seems it is 'yes' in this case), I think the edit should be approved.