The key here is that the Twitter is under no responsibility to facilitate your first amendment rights. The government, solely and exclusively, cannot limit your free speech; Twitter can do whatever they want, and are under no legal obligation to provide any service whatsoever to you.

I see. The user anigbrowl clarifies further below:

> Allowing the government to use your service [as its preferred way of communicating] does not thereby make it part of the government.