That's not what freedom of speech is about. You're free to say what you want in any country in the world, once. The first amendment is about protecting you from some of the consequences of that. Not specifically firing, but that's not his issue with Twitter either.
If this was a government censoring, banning journalists, stories and the media or even blocking the whole internet and punishing companies that criticize them or to stop the letter being published, that totally violates free speech.
Generally, It doesn’t apply to ‘privately owned companies’ Just like how these employees are free to criticize their employer it does not mean there are freedom of consequences and employers are just as free to fire their employees.
That is why they stay anonymous and criticize their employer just like what happened with Coinbase.
