That is true and I do login that way.
I was saying that for some things, Discourse does use email as the identity only such as SSO:
“Discourse uses emails to map external users to Discourse users” - Official Single-Sign-On for Discourse and it seems like there is not really another option.
Though thinking about it, if the user names on the SSO Source Site don’t change then Discourse could map users based on the username instead of email.