Here's the TechCrunch Article that just came out confirming:
https://techcrunch.com/2022/11/21/tumblr-to-add-support-for-activitypub-the-social-protocol-powering-mastodon-and-other-apps/
This is the opposite of today’s walled garden socials, where a post on one platform can’t be viewed by those on others.
It could provide Tumblr users with an entry point into the fediverse w/out having to use Mastodon. Matt had been thinking about how Tumblr could compete w/ Twitter: “I just want there to be an open alternative."