I think 8 was intended to stop that sort of thing, but maybe Apple thinks it doesn't apply?
8. The gatekeeper shall not require business users or end users to subscribe to, or register with, any further core platform services, as a condition for being able to use, access, sign up for or registering with any of that gatekeeper’s core platform services listed pursuant to that Article.
You are probably right, and now I wonder if this doesn't also apply to the standard Apple developer program.
I have not read the DMA but in the gatekeeper section of the official website the Core platform services listed are AppStore, iOS and Safari. Let's suppose that you single out iOs, why should I sign up something about AppStore to develop apps?
8. The gatekeeper shall not require business users or end users to subscribe to, or register with, any further core platform services, as a condition for being able to use, access, sign up for or registering with any of that gatekeeper’s core platform services listed pursuant to that Article.