This is way too strong of a conclusion to draw. They don't arbitrate all truth on their platform, they arbitrate it when not doing so could have dangerous consequences for a large number of people. There's a big difference between that and what you're talking about.
Everyone is an arbiter of their own platform. They do it when they take down spam on their platform, and they do it when they take down harassment on their platform. And now they do it when they take down calls to violence on their platform. I'd rather they do that than be a place where all of that festers.
> They have become the arbiters of truth on their platform.
That's the way it always has been everywhere. If you believe otherwise then you're being naive.
There's always a line and it's moved by the platform owner at their will when it suits them. These platforms are a big like cities with eminent domain, most people are aware of it but don't care until the city is taking away their property.