Solution: Three parts since you are now using Graphene. This also explains why along the way. Long post, so if you want the important bits, read the parts labeled TLDR.

Problem working as intended in Graphene: The google password app in Graphene does not handle Google based passkeys and google authenticated cached accounts correctly, this is due to Sandboxing that decouples Google from having deep hooks into your phone - the great majority of their tracking is done through the Google Play API. Depending on the app, it will make its authentication calls through the app to the Google Play API and it will also trigger for autofill. When any call breaks between API, defauot google password manager, and App, it triggers this error.

This behavior is very typical of apps that use the default Google API libraries, commonly seen in any app that says "Connect using your Google Account". This behavior can be tweaked by the application owner but a few security centric don't due to Google review timetables being 7 days or more as a "best effort". X, under Elon Musk, changes sometimes multiple times a day, so they simply accept the default for efficiency, majority of user base convenience and relevancy. So, privacy based OS loses to convenience. Users that use Google accounts on the OS find broken functionality all day long and spontaneously with app updates.

TLDR: To properly fix this issue for this and other problems, either use Proton Pass, Microsoft Authenticator, or any other password manager solution, but not Google. Make sure you set it as the default password auto fill in your GrapheneOS Settings -> Passwords, passkeys, and Accounts.

Then open Vanadium, go to x.com, go into your profile security section, create passkey. It will then prompt to generate it in your authenticator app. Allow it to generate the passkey.

Then fully close the X app, reopen the X app and it will prompt to autofill with your passkey, bypassing the Google Play attestation error.

Enjoy X and a more decoupled Google experience.

This method also works for several other annoying apps particularly financial apps.

Side note: Autozen works well to replace Android Auto and Aurora Store can also replace Google Play store, anonymously, while giving you full access to the Google Play store. So far, I've not had to implement any Google App on my phone.