That reasoning doesn’t work because an ordinary user doesn’t understanding what SSO even means.
Only what a new user sees is option to create new account and they use that. And gets very minimalistic page asking username and password.
Saving space is as bad reasoning, because login page isn’t any more fuller. First we should show SSO options and below those buttons old school way. Scrolling is not a new thing for users. We are still living in the world were janes ja joes are using google search of browser instead of giving url.