Let me point out the elephant in the room:
Comments requesting clarification/expansion can be answered by any passerby.
And also, such comments can be posted by any passerby.
It's perfectly possible to come across an ancient answer on Stack Overflow which was missing one little detail or variant application, and to comment asking for that. Here's an example.
You aren't obligated to respond, of course.
But you are obligated to Be Nice.
If the follow-up is just a tiny variation of the original question, it might be better off answered in comments (or an expanded, more general answer) than in a separate question.
Ask yourself: if I gave a sufficiently detailed answer to this question, and if this follow-up in comments were posted as a separate question, would it be closed as a duplicate? (If so, it's a reasonable follow-up for comments.)
- If the follow-up is related but really a separate question, perhaps it's already been asked before. If so, you can just link to it from the comments.
- If the follow-up is a separate question but hasn't been asked before, suggest that the OP ask a new question.
If you don't want to answer the comment, don't. No one is forcing you to. Communicating is part of life—what other pay is there for contributing to Stack Exchange?—but you're free to totally ignore comments on your answers if you want to.
It IS possible to respond nicely without doing the work of answering. For example, you might have said:
@crow, that's a completely different question. Please review the site guidelines, and ask it as such.
If you really wanted to be a bit snarky, you could have blandly added that they should do more of their own research:
...if your research prior to asking doesn't lead you to find your own answer, which it probably will.