It specifically calls the other options out as less suitable for SAML users or having other downsides.
Option two: "This option prioritizes handling username/password login, and may work better with password managers, but requires some Javascript handling that could be fickle."
Option three: "It simplifies the page, doesn't require a lookup of the domain, but could be clunky for the platform's SAML users."
Twilio is clearly aware of the other ways of handling this.
