May just be a gamble on future tech. IF federation is the future of the web (and I hope it is!) getting in early and helping shape it makes sense. Its also something of proven tech at the moment, so if they just threw this up fast to take advantage of twitter fires, it makes sense to use something that they know works as opposed to pulling a bluesky and doing it all from scratch. Also means there are more developers out there that are familiar with the tech.
Oh sweet summer child.