They haven't specifically said anything, but they have directly compared the ID to phone based payment card systems, which on the google side do rely strictly on a google-blessed android build[0][1][2].
It's also incredibly popular in the security industry (I know, I work in it) to claim that every possible app in existence must:
* Obfuscate
* Do root detection and refuse to work
* Detect attempts to attach a debugger, and refuse to work
* Detect running from a VM, and refuse to work
* Do certificate pinning (although as an industry we've stopped recommending this bullshit practice, although we still insist on it for some things)
* Prevent screenshots from being taken
* Force you to re-authenticate using biometric ID every time you look away from the app
* and... break at the slightest hint of a non-standard build of android
So I don't have high hopes, because the company I work for does work for the UK government, will likely be picked to review this app, and inevitably all that shit is what we'll recommend (although I hope I won't be working here by then because I'm just sick and tired of cargo cult / checkbox security).
[0]: Not because of any specific feature, but solely based on signing keys.
[1]: I believe specifically you have to license GMS integrate them into the build, which e.g. GrapheneOS does not do.
[2]: And no, GOS's sandboxed google services don't fix this problem, Google Pay will still refuse to work.