The obsolute worst thing about the Fediverse (Mastodon et al) right now is the onboarding process. You have to "chose an instance" before you can even start understanding what that's about - and if you chose wrong you might find your instance gets switched off leaving you high and dry in the future.
Now imagine Tumblr ships ActivityPub. Every Tumblr user can now follow any Mastodon user, and vice-versa. The Tumblr ecosystem gets a whole bunch of new high quality available content. Mastodon enthusiasts can tell their skeptical friends "go sign up for a Tumblr account and follow me from that".
Or... if you're launching something new, why would you launch from a state of zero accounts when you could launch with Fediverse compatibility and have ~10m accounts that your first users can start following straight away?