The app is available only on Android (and maybe iOS) and requires Google/Apple attestation.

There's been extensive discussion about this, but the developers refuse to even fully acknowledge the problem.

Is there a “the App”? The text I referenced talks of “a Reference Implementation”. That suggests, to me, that there will be multiple apps, leaving open ones for other platforms than smartphones.

And yes, it remains to be seen whether, as they say “alternative mechanisms for storing and presenting proof of age” that “may be considered for future versions of the solution” actually will actually surface, but their argument “The solution relies on a device-based proof of age model, leveraging widely available mobile devices such as smartphones and tablets to store age attestations. This approach supports the goal of rapid deployment and broad accessibility.” has merit.