Touchy subject but I'll weigh in.  I think the "line" is restricting visibility.  Therefore downraking false theories is censorship, but not illegal if not done by the government.  But flagging posts or labelling them with additional information is not censorship, it is literally "more speech".  When done by a platform, it is the platform speaking, which they have a right to do.

I prefer a nostr system where we all flag each other, and we each get to choose whose flags we listen to or care about or display.