"Never" is a strong word. API translation is a technical hurdle, but rarely an insurmountable one. If Blue Sky wanted to add an ActivityPub interface to their platform, they probably could.
This issue isn't technical per se; it's a matter of priorities. Blue Sky doesn't want to federate with Mastodon/Threads, because they want users to switch to their platform.
If something has hundreds of "centralised" platforms owned and run by a diversity of different people and spread all over the world geographically, then that's "decentralised".
I can't really think of another way in which something could be decentralised.
With ActivityPub, there's nothing stopping you hosting a server literally just for yourself. It wouldn't get much more decentralised than that.