And even if this is somehow not an ideal use of the database, it's certainly far from "terrible", and to conclude that the programmer who designed it was "clueless" is insulting to that programmer. Even if that programmer was you, years ago (as in the blog post).

Moreover, unless you can prove with experimental data that the 3rd-normal-form version of the database performs significantly better or solves some other business problem, then I would argue that refactoring it is strictly worse.

There are good reasons not to use email addresses as primary or foreign keys, but those reasons are conceptual ("business logic") and not technical.