Cool to see you perfectly got the point in the end! I wonder though, were you confused by the README? What made it clear for you?
I think my confusion was in trying to frame things as parsing VS validating. While I now appreciate that use of word, now that I understand, it also caused my biggest source of confusion.
That's because I think most people think of parsing as conversion, like I turn a String to an Int. Where as in your case, you're simply wanting to tag a type as having been validated, but you don't really convert the type itself, so you simply wrap it in another type in order to tag it as having been validated simply because the language offers no other way to tag the type with meta-information for the compiler to assert statically.
So because it seemed more like you're just wrapping the input, but still all code will be using the input value as it is, extracting it out of your wrapped type, the idea that you were "Parsing" and not "Validating" well just confused me.
If you wanted to go further you could start going the "correct by construction" route - having a data type that enforces more invariants. For example, you might store the recipient name, domain name, and tld of your email in separate fields. Then your parser would more obviously be a parser. I think of the "mere tag" type as a kind of degenerate case of this, rather than something totally separate.