The point is that e-mail vendors (e.g., Google, Microsoft) have dedicated far more resources and effort to securing their e-mail software (e.g., GMail, Hotmail) than start-ups will, or can, dedicate for their own authentication component. (For example, how often are all GMail passwords been stolen?)
I am suggesting that the authentication process for e-mail is far more secure than what a start-up could produce to authenticate for their own site. I am not suggesting that e-mail is NSA-proof.