"Free speech absolutist" does not mean "absolute free speech", you're misunderstanding the premise here. The term does not mean that anyone should be allowed to say anything they want at any time.

The phrase means that we should permit any LEGAL speech. Where "legal" has tons of historical precedent and can be decided by the country.

We've seen time and again, that if the ability to speak freely isn't a priority, then censorship grows quickly. If you don't believe that there is a ton of censorship happening on these platforms with a specific set of biases (the biases that the employees of these companies carry) then I would say that you might not be viewing the situation with an open mind.

This is becoming a problem because the Internet has become the new town square where people learn what's going on in the world and talk with each other. If the people running these platforms are allowed to suppress speech that they don't like and promote speech that they do like, then they wield an incredible amount of power. This power is rife for abuse, both by people inside the corporation and within government.

You are correct though that 4chan is a gross cesspool, though its one I believe should be allowed to exist simply because I believe freedom of speech is important. The problem with that site is obviously that it's anonymous. Coupling anonymity with free speech is a recipe for bringing out the worst in people, but a system where peoples' identities are out in the open and where they can speak freely is good in my opinion. We need more free speech and we need to engage with our fellow countrymen and find common cause, otherwise this insanely polarized partisan situation will continue to get worse.