I thought that's a soft-lockout that's supposed to tell you to stop trying to comment on a topic. If you're persistent and really want to, you can after a minute or two, but if it's not that important, you go "meh, I guess not" and go on your merry way.

If my understanding is correct, they disable the “reply” to limit more comments. On every comment beyond a certain depth, reply is disabled for some time immediately following the comment.