You can't reply to topics that are started as comments sections for an article or mod page. The comments section uses the forum as a backend, but commenting in both the forum thread and the comments section breaks the formatting really, really badly.
If you want to comment on a thread that has "you cannot reply to this topic" at the top instead of "locked", then just go to the very first post and there should be a link to the article/mod page that you need to comment on.
Ah, had no idea that it is working like that. Thank you.