I admit that this may be a fuzzy edge case that I'm not completely familiar with, but I think the objection is not that Congress is completely exempt from the act but that the president had the executive branch issue a rule that allowed the government to subsidize congressional health plans, where this is not allowed in any other business.

This is technically separate from the waiver objection.

I know there's an objection here, so if you can find a reference to it I would appreciate the clarification.