I think this is referring to the FedCM api [1]

It works with providers other than Google, just no one else has implemented it yet. Google's One Tap library tries to use the new api, and falls back to the classic One Tap popup when using a browser that doesn't yet implement FedCM (notice how the chrome built-in one says "sign in with google.com" rather than "sign in with Google" like One Tap normally shows)

Mozilla are working on implementing it, but it's a pretty complex system so it'll take time. I presume Safari will as well if it gets popular enough

[1]: https://developer.mozilla.org/en-US/docs/Web/API/FedCM_API