That's opt-in, mangles usernames, and on top of that quite a few people on Mastodon seemed allergic to the very idea of bridging/federation the last time I looked into it.
So? It's just an example. I'm sure you could do it in a cleaner way. They use different protocols. If you can run your own server and connect with open source tools, it's decentralized. Though of course that doesn't mean a decentralized protocol isn't highly centralized. See email
I thought Threads only interoperates with Mastodon/the fediverse in some limited capacity. Did I miss some Bluesky integration announcement?