It isn't, it's not enshrined in law, de facto does a lot of work here. IANAL, but I'm pretty sure such a requirement will not hold up in court either. Besides that, the developers of the reference app have stated that national apps do not have to require strong integrity from Google Play Integrity. It seems like they took the standard platform path either because they did not have time the time or knowledge to do anything else.

At any rate, I'm optimistic that it won't require passing strong integrity in my country. Age verification will be added to our national ID app (DigiD), which does not require passing strong integrity, even if it is used for more security-critical applications than age attestation.