The argument for it being anticompetative is straightforward: apple is leveraging it's position in the mobile Phone space to push it's way into social login "markets", or equivalently hurt others in that space.
but they aren't removing choice, they are only adding choice in their approval processes, and leaving it to the developers to decide to remove choice when they get social login fatigue but want high engagement and low friction. Looks like they dotted their I's and crossed their T's on this one.