I love how this article cites Brad Frost's "Don't Get Clever with Login Forms"[1], yet at the same time advocates specifically for doing one of the "don'ts" listed in that cited article ("don't split login across multiple pages").
Reading through Twilio's article, I still don't see a satisfactory explanation why they did it this way instead of just presenting both fields plus a list of buttons for other authentication methods (like what they even acknowledged to be the norm, citing examples like Pinterest and Twitch). If you're trying to dynamically choose the authentication method by looking up some preference associated with the user's email address, then you are exhibiting the textbook definition of overthinking the problem.
If users want to login with Google/Facebook/Twitter/Microsoft/etc. credentials, provide a button for it and call it a day. Those login methods will automatically (in all likelihood) provide an email address for your app to tie to its internal "user" representation (though you might have to explicitly request that particular permission in the OAuth or SAML or whatever flow).
Reading through Twilio's article, I still don't see a satisfactory explanation why they did it this way instead of just presenting both fields plus a list of buttons for other authentication methods (like what they even acknowledged to be the norm, citing examples like Pinterest and Twitch). If you're trying to dynamically choose the authentication method by looking up some preference associated with the user's email address, then you are exhibiting the textbook definition of overthinking the problem.
If users want to login with Google/Facebook/Twitter/Microsoft/etc. credentials, provide a button for it and call it a day. Those login methods will automatically (in all likelihood) provide an email address for your app to tie to its internal "user" representation (though you might have to explicitly request that particular permission in the OAuth or SAML or whatever flow).
[1]: http://bradfrost.com/blog/post/dont-get-clever-with-login-fo...