From what I've seen so far, Activity Pub does allow multiple apps per use so individuals can choose an app of their liking. For example, I'm using Pleroma instead of Mastodon and the interop with Mastodon instances has been seamless. It's early days exploring for me, though.

This issue of bridging the IndieWeb and the Fediverse is fascinating. I'd think it would be more naturally collaborative, but there's a lot to sort out yet.