The FSF picks some weird gripes to focus on. At least they're no longer arguing Bluesky isn't federated. Though this claim is false:
Hosting your own Bluesky "Personal Data Server (PDS)" and relay is technically possible, but generally far more difficult and resource-intensive than spinning up an instance of Mastodon,
This is just not true? ATProto is a much more efficient data transmission means than ActivityPub. Running a Mastodon server (I've done it) means hoovering in and storing massive amounts of data even if you only have a single user, for discoverability. A PDS by contrast is like hosting a website. You ping the relay when there's changes, and it comes by and sweeps up the new data. New relays may periodically come along to spider your content, and old relays might re-spider from time to time, but the load from spidering very low. The frontends in turn query the relay (basically, they ATProto-"google" it), and when they know where/what to fetch, they fetch it from you - again, if you don't have much hugely popular content, you're not serving much either, just like running a local website. And your local stored content size is again very low. Unlike Mastodon.
Running a frontend (AppView) - for example, what a website, an app, or whatnot uses - is also super-lightweight if you're small. You're just "googling" and fetching whatever content you need to display to your users. Little user activity = little server activity.
Now, if you want to run a relay, that's basically like running a web spider. You're probably not going to do that in your bedroom, but it's not expensive. In the old architecture, the cost of a server that could function as a relay for the whole current network was about $150/mo. But after some architectural improvements, it's now more like $35-40/mo. And that's to relay all of Bluesky, but you can also relay just a subset. AppViews can use whatever relays (aka, "search engines") they want to to collect data.
It's a very efficient architecture, particularly for small users. Just think:
ActivityPub (Mastodon): Forums. Everything all in one, each ruled by a Local God administrator who can do whatever they want to you and your data (delete, alter, etc - and you can't move it off their server), that are constantly running data syncs to each other, including the really big servers.
ATProto (BlueSky); The internet. PDS = Website. Relay = Search engine backend. AppView + Site/App = Web browser. You can move all your data to a different "website" (PDS) or switch "search engines" (AppView) at any point. (In the analogy, moderation (labelers) is SpamHaus, just at the data level, not site level - they label the ID of a given piece of data or your account, and any AppView / user that trusts what a given labeler has to say can react to different labels as they choose) (anyone can run a labeler too if they want).