Wordpress doesn't have ActivityPub built in, it's a plugin in beta currently. Without AP, there is no client that can pull in website feeds and provide discoverability between WordPress sites, Mastodon posts, etc.
People love to reinvent the wheel and claim it's a whole new thing. No ideas on the web have really been innovative since the bubble popped. The innovation has all been on delivery and execution (not wanting to discount any of that).