Ah I forgot Apple advertises managing SSO as a feature of iOS and not an external service like sane people would.

Well. I guess they'll have to choose between opening it up like every other company does or acknowledge that it's a separate pay for service then.

They do a lot of that kind of thing and my answer for all of it is the same: Open it up to everyone or acknowledge it's a pay-for cloud service that has nothing to do with the actual phone OS. If people have root they can (and will) develop their own services that won't need that which would comply with the DMA.