That’s surprising to hear. I have zero DB experience, not am I a developer, and my default assumption is that some UUID maps to object / entry containing email address (and all other attributes).
When I was in college, academics were just starting to grudgingly accept artificial primary keys (like UUID or auto-incrementing integers) as a concession to reality.
To a modern developer natural primary keys sound insane. Artificial keys are faster in almost every way due to index size. You’re storing compact, fixed-size binary data instead of strings.
Socials change. Emails change. Seemingly natural, unchanging things change and mess up all the foreign key relationships.