If I understand the model of the hypothetical app referenced above, they send email containing confidential (already, this seems problematic, but let's ignore that) "trip" information to some email address. Within that structure, then sure it's problematic to have to require two different email addresses. Except, you don't have to do that. Don't require an email address to start using the app. Let users enter whatever confidential trip info they want, keep a reference to the resulting DB records in a cookie, and only require an email address when the user wants to, uhh, get an email sent.

Having typed the foregoing, I realize now I'm basically just saying "don't use any third-party sign-in, including SIWA". Maybe it's fine that the ecosystem doesn't always cater to apps of questionable utility...