While it perhaps seems a bit heavy-handed, it's an easy way to ensure people notice the channel has moved, even if they haven't been following the news, and thus minimize damage to your community. I don't see how this action is "contempt for the community".
"How many people actually moved?" .. I'd presume everyone who wants to be in that channel and who has noticed that it has moved. Why would they not?
Also note that this example seems to be the exception, many channels that got hijacked merely mentioned the move in topic or even just mentioned libera in topic without any decision to move being finalized (though the hijackings did a great job at finalizing those decisions ;)
It's easy to say the community moved when you kick and ban everyone from a channel and create a new channel on a different network. But how many people actually moved? Did they just damage their own community over this?