The argument here is kind of hard to follow. Who is the "owner" of the phone, "the user" is also mentioned and it is not clear if these two are the same. Is the owner of the phone in the controlling-software sense, Google, or is it the end user? Both fits, and both are commonly used.

Because if it is the end user, the strong version of the argument would be as follows: The end user signs a document, baked in is an attestation that Google guarantees that this device is an approved Android device with a clean boot chain and a Chrome web browser. Then the end user contests the signature in court, either because they didn't understand what they signed, or they did not sign it at all, or did it under threat. How could the attestation help here?

I do not have experience with all EU countries, of course, but more than one, and nowhere is this an issue today. Countries use a wide variety of electronic identification, from soft certificates and mobile phones to smart cards. But as far as I know, all countries accept signatures made even with normal Windows PCs. You can contest a signed document in court for a multitude of reasons, but that's not specific to electronic signatures.