Nobody has argued that Twitter cannot ban people from their platform at will because of what they say. The argument has always been that it's a bad idea for them to do so for a whole variety of reasons:

1. The inevitable inconsistency in application creates hypocrisy, which makes people upset.

2. It attracts political attention if/when the enforcement is politically biased.

3. It costs large sums of money that could be spent on other things.

4. It isn't actually necessary.

5. Public forums in which ideas can duke it out are essential for a healthy democratic society. Someone needs to run them, so if you decide to create an explicitly public forum open to everyone then you have a moral duty to protect and implement free speech policies

etc etc. Not an exhaustive list by any means, just a subset of the arguments that can be mounted.

But note that none of these apply to the case of employees criticizing their employer.