Is the downvote/comment approach still the correct approach here?
Yes, that is the correct approach, in my opinion. You generally should not edit such an answer, unless the incorrect nature of the answer seems to be an error of typing or the like. In your case, it seems that the incorrect answer is exactly what was meant by the poster, so editing it to fix it would violate the principle of not changing the meaning of a post with editing.
A significant problem with editing a question's content for correctness is that it circumvents the voting system as a way to judge content. Yes; the voting system can be wrong. It's an imperfection, because people are involved, and the style and other human-oriented factors can affect it. But overall, it works moderately well as a way for a discerning reader to gauge posts.
Editing for correctness instead of voting/commenting basically lets a single person override the whole system, to a degree. It's that one person saying, "the community is wrong, and I am right." Maybe that's true; but there is no good way to judge that.
What if the editor, for example, is the one who is wrong? How do we judge that? Again; we already have a system in place for that.
I do not have any desire for "furthering incorrect information", but this is the Internet. Users with short attention spans are liable to accept lots of incorrect information. For example, what about a user who skips the "correct" answer because it seems like too much work, and instead goes for a really brittle, bad solution that they 'like' better?
Mostly, I think it all comes down to the principle that the voting system works pretty well on a broad level, and we need to be careful about individual users trying to override that, because there is no system to make sure they are right/wrong. Posting their own answer is the best way to do that, and have it judged. Perfect? not at all... but not so bad in aggregate.