I think you're confusing the implementation of threads with the feature itself. Emails (and mailing lists), forums, reddit, HN, etc has shown us that threads are great. IMO they make discussions less ephemeral which is a good thing for slower communication methods. So hopefully the use of this tool is less "chatty" and less stream-of-consciousness.

I haven't used Quill, but judging from the screenshots, the implementation looks nearly identical to Slack's. So when they say "everything is a thread", it seems safe to assume they mean "we like Slack threads so much that we made our entire app behave like them". I'm not here arguing against the concept of nested reply structures in all cases.