They don't like that there's a chance their post will remain indexed on Mastodon even if they delete it.
I understand the integration concern, but it's adorable that they think anything gets deleted from any of Zuck's platforms.
But more to the point, that's an interesting read, and I'll have to compare Phanpy to my current choice: Fedilab. Like they said, there's lots of clients and possibilities for clients, since the Fediverse code is fully open source.
I don't care to integrate with Threads, since there's plenty of people talking about plenty of topics on Mastodon (and all it takes to find them is following a hashtag), but for the people willingly exposing their feed to Zuckerberg, I'm sure they'll be happy to know that the Fediverse still has a lot to offer.