Arguing that Apple has a de facto monopoly (or oligopoly) on phone OS and related services is a defensible argument.

This, however, is almost the opposite. Apple isn't banning any other authentication mechanism. Authentication services can still compete and app developers can even listed their preferred auth methods preferentially. Apple is only saying that Apple's service has to be given a chance to compete as well. Apple is certainly using its market power to enforce policy, but if anything it's pro-competitive (even if it benefits Apple itself).