If they had SSO sign in to their admin panel (trusted device checks notwithstanding) the oauth access would be useless.

Vercel is understandably trying to shift all the blame on the third party but the fact their admin panel can be accessed with gmail/drive/whatever oauth scopes is irresponsible.

That's a low-leverage place to intervene. Whether or not the internal admin system was directly OAuth linked to Google, by the time the attacker was trying that, they already had a ton of sensitive/valuable info from the employee's Google Workspace account.

If you can only fix one thing (ideally you'd do both, but working in infosec has taught me that you can usually do one thing at most before the breach urgency political capital evaporates), fix the Google token scope/expiry, or fix the environment variable storage system.