Not yet, but they’ve said it will be. Personally I’m excited to see “mainstream” social media using ActivityPub, but you’ll find that it’s quite a divisive issue.
I’ve said this elsewhere, but I don’t really agree with the article’s premise about what happened to XMPP.
XMPP was a niche thing before Google appeared, and it remained a niche thing after Google left. That said, it would have been much better had Google continued to support it!
Tumblr said that too but never did it actually. It's not so easy to join a network where the skills of any single "kid" could be compared to your 1000+ employees company.