> Federal documents don't have authenticated addresses. The view seems to be that only state agencies are capable of verifying you actually live at the address on your ID (See Real ID for more context).

Real ID isn’t about this. Federally issued IDs like passports and NEXUS or Global Entry cards can be used in every context where the REAL ID Act’s requirements apply to state-issued driver licenses and non-driver IDs, without any exception I’m aware of, even though these federal documents are not proof of address.

But sure, your explanation might well be the justification behind this USPS / Login.gov policy.