Bluesky is almost fully open source, as Mastodon is
And sure, Bluesky is currently operated by just one company. But federation may change that. And many Mastodon instances and services are also operated by companies.
Refusing to federate with Bluesky because of "corporations" should lead the people who have this stance to also disengage with significant portions of Mastodon. But I’m assuming they don’t.