SOLUTION for me (public answer):
Instead of the homepage login, go to: www [dot] delta [dot] com [slash] skymiles [slash] login

I've cross-posted this here and elsewhere, and with the exception of one poster claiming I've been spreading "misinformation", this is a major issue that is either Delta's login engine, or third-party antifraud (Imperva Incapsula, like Hertz uses).

Private answer (misinformation, according to others) - I had a Delta friend run a full analysis internally on my wife's account.
It appeared to be a mismash of iOS 26, Windows 11, macOS -- coupled with their broken identity engine FOR ACCOUNTS WHICH HAVE MULTIFACTOR enabled. (which never works, anyhow - I've never gotten an email or text message code - ever).

But... time to merge threads? Locked out of app - Delta confirmed issue