The Credential Management API is more like a API in to the browser's password manager. A page can save a username/password/token into secure storage (which the browser can sync to your other devices) and later retrieve it. This is nice because we don't have to rely on autofilling form field heuristics.
You're thinking of Google's One Tap, which is just a JS snippet that publishers include on their page, and has nothing to do with browser APIs. (The OP shows a screenshot of One Tap, which is confusing.)
These uBlock filters will make those sign up with Google popups go away entirely:
You're thinking of Google's One Tap, which is just a JS snippet that publishers include on their page, and has nothing to do with browser APIs. (The OP shows a screenshot of One Tap, which is confusing.)
These uBlock filters will make those sign up with Google popups go away entirely: