They don't have to actually be a 3rd party identity provider, just the user has to find it plausible that they might offer 3rd party login. Which, to be honest,

They don't have to actually be a 3rd party identity provider, just the user has to find it plausible that they might offer 3rd party login. Which, to be honest, pretty much any big or even medium-sized tech company might be doing these days.