They're originating content.
On a "platform" that means the creator of the content (i.e. user) is legally liable.
On a curated platform / publisher (where editorial control and content creation is performed routinely) the platform/publisher is liable.
For example, here, 8chan is not responsible for user content, because they do not curate beyond removing illegal content (i.e. CP) nor does the platform routinely produce its own content; it is a tool for users to use, and individual users are liable for any posts with content that could expose them to legal repercussions.
Not so with a curated platform. Even less so with a publisher (who loosely acts as a platform) but is actually creating content (tweets) regularly. The publisher itself becomes liable – kind of like if NYT were to routinely post libelous info in their paper – not just biased stories, but blatantly false info not even based on any real sources.