From the Terms of Service,

If you sign up for an account, you will be able to submit questions, answers, comments, and other types of posts, some of which may include images, video, or other media (collectively, "User-Generated Content" or "Content").

By posting here, you license any such Content under the terms of one of (in increasing order of precedence) (a) the site default license at the time of creating your post; (b) the license of the category in which you post, or (c) the license you choose from the available list at the time of creating your post — the "Applicable License".

Emphasis my own.

IANAL, but I believe these sections imply that comments do fall under the same rule as other "User-Generated Content". This implies that, given (c) does not currently exist for comments, comments would fall under (a) or (b): the default license of the category or site.

I wonder if there's a more explicit way to word this (and whether it should be indicated at the top of a comment thread to avoid ambiguity).

For the author of a post, it might seem reasonable to assume that (c) indicates their comments will be under the same license as they chose (actively or passively) for the post the comment thread is under. It might also seem reasonable to assume that (b) indicates that the license of the category applies since the comment did not have a choice made.