Why does Youtube being a platform means they have to post everything? Are the anti-vaccine video sites forced to post pro-vaccine videos?

I only recently learned that this was a myth. The core of S230 is:

    No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider.

As long as YouTube isn't creating the content they are protected.

They are both and the distinction is mostly meaningless.

Publishers and platforms are not mutually exclusive concepts.

Old terms partially describe a modern concept...