Bluesky doesn't use the ActivityPub protocol, instead they have their own Authenticated Transfer (AT) protocol. That protocol is open source, so theoretically it might be possible to build an ActivityPub to AT protocol bridge. That's a deep maybe though, there might be wildly different approaches taken in each that would make it hard to build sensible mappings from one to the other.
I tried to look into it, and have asked others to explain, and I'm not sure it's actually decentralized. It's not open source, and I don't think anybody can just create their own instance (or else there would be more instances by now)
I'm not sure I'd call Bluesky decentralized. It theoretically has the ability to have multiple instances that talk with each other, but I don't think anyone besides Bluesky has actually set one up.
Everyone using it is on one instance and it can't talk to Mastodon, Lemmy, etc.