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.