My understanding of the new Gmail OAuth procedure is that that our does require the account be added to the device. You would need to use an app password and the “less safe apps” setting (or whatever it is called).
While I have a Gmail account that really has no other need to be an on device account, I opted to add it, as K-9 Mail was the only reason I still had app passwords and “less safe apps” enabled on any Google accounts.
I can confirm that this is a new issue on my pixel device.
I am no longer able to modify my fetching mail server configuration and update the App Password that was being used, as I am now forced to the Add Google Account to this Device screen, which is not something I am able or willing to do.
Sadly, this will mean the end of k9 updates for me and I will downgrade until this is resolved.
You said it is a new phone. Have you by any chance accidentally tapped on “[try/use/activate] enhanced security” when initially setting up the Pixel device?
If “yes,” you cannot access your Google account (or parts thereof, such as Gmail) without having the corresponding account attached and activated on your device. The setup MUST be completed through Chrome for some reason.
You can check in your account if you have enhanced security active. Alternatively, you can try installing an app from an APK/AAB file or from F-Droid. If installation is refused due to enhance security (not due to untrusted source! I literally mean due to enhanced security), the enhanced security is active on your account. You can only install “safe” apps from Play Store (even from there some apps will be blocked from installing).
You can deactivate enhanced security in your account. It will take roughly 24 hours to synchronise through all Google services.