Most of the time it is in my opinion and observation possible to give a more detailed answer.
Points that can often be addressed include:
- Suggesting minor improvements of the presentation or variations.
- Giving a high-level summary of the argument.
- Mentioning points where details might be added (even if not strictly needed).
- Putting the argument in a larger context.
- Suggesting alternative arguments.
I do not mean to say that all the time all this should be done. I only mean to say that almost always at least one of this can be done.
Doing so will not only make for a more interesting answer, but can also raise confidence in the veracity of the answer and thus the proof in OP.
Anybody can post "This is correct." If this is of any value at all depends mainly only on who said it and possibly the score.
This is not optimal for an answer. An answer should be able to stand at least a bit on its own. It should not just give the binary information in/correct but also address the "why."
This is a bit like when conducting a session of a course where students present their solutions of homework to the others. I think, normally, an instructor should make some remarks during or after a student's presentation beyond "This is correct."
Thus, I do not consider it good practice to post a "This is correct." answer, at least not early on. (As a comment it is fine.)
The point is precisely the question will appear to be answered while actually it does not have a detailed answer.
There is also a practical concern. Very short answers generate an automatic "low quality"-flag. First, treating these flags is work. Second, somebody answering in such a way should at least pay attention to make very clear that the answer is not a "misplaced" comment, which is not always the case. Leading to confusions, and still more work.