How does that person think ATProto would prevent monopolization? ATProto is effectively controlled by a singular corporate entity. It already is a monopolized protocol.

Bluesky says they created ATProto because account portability is a problem on ActivityPub - which it 100% is. But account porting is something that's still being worked on for ActivityPub. If Bluesky actually cared about a decentralized platform, they would have contributed toward ActivityPub's development by helping to create the account porting tools. Instead, they created a new network that they own and control, and have gaslit their users into believing is "decentralized".