Currently Champions can still comment on closed questions and discussions. However, the "Closed" label isn't particularly prominent, making it quite easy to overlook and accidentally leave a comment on a closed post.
I'd suggest disabling the comment/answer option for everyone, including Champions, once a post has been closed.
Since Champions already have the ability to reopen posts, they could simply do so if they want to continue the discussion, which would then enable commenting for everyone again.
This would help prevent accidental comments on closed discussions and make the overall experience more consistent.
Alternatively (or additionally), making the closed status more visually prominent could help avoid confusion. 🙂
