From what I've read when they first went public about it, it was because of #portability which don't exist in the #ActivityPub protocol. When I informed them about the #NomadicIdentity powering #Hubzilla (and #Streams today), they said they weren't aware of it (at least from what I can recall), so they'll look into it.
The second reason is moderation. Which, if we only look at AP protocol it's pretty much basic, if any. But if they were only aware of Hubzilla's #Zot protocol before they started their project, they probably would've used it since moderation and permissions are awesome in Zot (far better than what #Bluesky / #ATproto currently have today).
And well, they already started their own project and set up their goals, so they pushed for it.
At least that's how I understood it. Not necessarily accurate.