An ActivityPub feed for a blog is absolutely doable, it just requires work. For example, a quick search turns up an ActivityPub plugin for Wordpress. Heck, micro.blog supports it natively. There’s also bots out there that’ll auto-syndicate posts from an RSS feed to a Mastodon account. Unfortunately it’s just not very widely available or adopted (yet!).

(https://b-ark.ca/a2UG4Q)