They aren't using ActivityPub, what Mastodon and Lemmy use, they're using their own new protocol called AT (Authenticated Transfer). So it's less that they don't plan on federating with anybody else, and more that there's nobody to federate with. Maybe somebody else might pickup AT in the future, but AT is still a work in progress and there isn't a lot of incentive for anybody to do so yet.

I believe the idea is that they're not federating with any other existing site, they're creating a new variety of federation with themselves as the first member, with people being able to set themselves up as additional independent nodes in that federation.