My understanding is that bluesky has full support for federation on a technical level. I would agree with describing it as fake federated, because probably over 90% of users/content are on the flagship instance. If the flagship instance defederates from yours you've probably lost most of the reasons you had for joining bluesky and will want to switch. If they change their mind entirely about federation, even the users you followed that weren't on the flagship themselves will probably be switching over, so you've essentially lost your entire community unless you switch yourself. When one instance has most of the userbase, the whole network outside it can be killed if they decide to switch off federation.

This is actually why the join-lemmy site is coded to only display instances with under (iirc) 30% of the user share. Right now that's .world that won't show until their usershare drops under.

As far as why they would do that, I assume just as another way to market it? Probably hoping to syphon off some mastodon users or selfhosters at its inception to get the userbase growing.