I noticed an edit that was really adding significantly to the author's original answer. It was a whole new thought or addition to the answer. Is this what edits should be used for, to add a new thought or addition to an answer that wasn't from the original author?
I've always felt like we have too many "and on top of X's answer" posts, and per my comments on improving the signal to noise ratio I'm a fan of more agressive editing.
How I've been working is that if I feel like I can "save" a poor quality answer that has a decent idea at it's core by expanding it in detail, like Why is my trigger receiving 0% code coverage when the test class runs correctly? then I will. It's not a substantial change from what the initial author intended to say, but it does elaborate and link to the source data he was likely thinking of. Likewise if I see an answer that's nothing but a link to a relevant, useful source I'll edit edit the post to add extra info from it, or leave a comment requesting the initial author does. The net result is a pretty dramatic edit, but I don't really see that as a bad thing here.
Remember, the FAQ does after all say
If you are not comfortable with the idea of your contributions being collaboratively edited by other trusted users, this may not be the site for you.
Substantially changing the author's meaning is, as far as I'm concerned, the one thing off limits, although where you draw the "substantial" line is hazy.
Offhand, I would say, "no", with limited exceptions. If Salesforce changes something that invalidates the answer I could see adding an update section at the top of the answer, detailing what changed, but leaving the rest unaltered. Maybe for clarifying something tricky would be acceptable.
Going by your explanation and not having seen the OP, though, it's hard to say if the edit was warranted.