https://fedidcg.github.io/FedCM/#browser-surface-impersonation says:

In addition, the FedCM UI is deliberately constructed to not prompt the user to provide additional information, such as username or password.

A specification should not make claims about browser-specific interface aspects, except to the extent that we can agree on common requirements.

In this case, this claim doesn't appear to be a good one. It would seem to be possible to inline a login flow in the UX if a browser wanted to. That might not be good UX, but I don't see any requirements that would prevent it. Nor do I think I would agree to such a requirement if one were proposed.

Instead, I suggest that this be framed in terms of requirements on browser UX that need to be met in order to maintain the security goal.