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 seem

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.