Comments on issues and PRs are calles "posts" everywhere(?) except when deleting them, then they are called "comments": See forge.org section "Editing Topics and Posts". The command is also called forge-delete-comment. The forge-delete-comment command uses forge-comment-at-point instead of forge-post-at-point. forge-delete-comment is the only user of forge-comment-at-point, which is a good thing as that function is broken. It returns t instead of the forge-post object at point.
I think I got all of that right :)