> This is a weird case of private companies getting together and voluntarily enforcing the supremecy of the state.

It's maybe weirder than that, because of the involvement of the president. Even if you believe the president was entirely uninvolved and unaware, the purpose of the insurrection was to keep a leader in power, not remove one. The actions of tech companies are both a challenge to the supremacy of the (current administration of the) state and a way of enforcing the supremacy of the state.