This is not true.

The App view, relay, PDS, and mobile app, code is all open source. Anyone can spin up their own version if they want.

If you want a complete copy of bluesky (reflecting the bluesky post firehose) it's prohibitively expensive, but you can spin up your own version if you want.

The only thing that's missing is bluesky federating anything other than PDSes by default, but you can 100% go in the other direction.

A good overview is here: What bluesky is and what bluesky is not

It’s not something where any one person can really host their own instance long term — though people still do (for now). But AtProto is designed so a reasonably-sized company (or maybe a well-funded foundation) can host their own instance and either make a clone or do something novel.

As I understand it, the core difference is just the scale of what you’d have to host. ActivityPub only downloads what users on an instance interact with so you could easily run your own one-person instance on a home computer. A BlueSky instance downloads everything so you’d essentially need the scale of BlueSky (which is already in the terabytes)

The upside to AtProto for users is that your username and content are all portable and you can switch providers (or even use multiple) and not lose anything. ActivityPub’s downside is that it can leave you at the mercy of your admin. Not a big deal on the main instances but there’s been some drama moments where some admin freaked out and those users essentially lost their account.

AtProto/BlueSky was originally envisioned (pre-Musk) as Twitter being semi-decentralized so it’d essentially be the hub of a wider ecosystem. But, obviously, the world’s worst truck designer had other plans.

Yes, it’s not a full web3 app, but I like that these apps are heading in the right direction.

However it's better than Twitter/Instagram. I can control my feed!