It is, and they can do what they like! I have moved to Mastodon without too much fuss and I'm enjoying it so far.

Does that mean it's a good idea, that it's not rank hypocrisy, or that it's in any way in the spirit of a "marketplace of ideas"? No.

Let's say I was a person rooting for Twitter under Musk to succeed; even viewing it through that lens, this seems really really super dumb, and I can't imagine people sticking around if they can't promote their work elsewhere.