I mean that looks a bit more like wordpress expanding their platform/ecosystem to get more engagement from mastodon (AFAIU, they've implemented user based federation only) ... which is all good.

But it's not the same thing as fulfuling the fediverse promise of a single ecosystem in which you have many options/possibilities to create the social graphs and interactions you want. In this case, something like a platform/plugin etc where any fediverse account (lemmy, mastodon, etc) allows you to subscribe/follow a blogger through a subscription, which is paid if desired, all without really having to leave the fediverse and be bound to the whims of any particular platform/company.

I don't know much about wordpress so maybe all of that is there already?

But, if there hasn't been some migration from substack to wordpress, then I have to presume it's because that ecosystem doesn't provide the same thing that substack does, and which I suspect the fediverse with its more social-media inclined platforms could provide if it had native and well-integrated blogging platforms or features with the ability to have limited subscription-driven access.