[masklinn]

TFA does explain what they mean:

> in my mind, the difference between validation and parsing lies almost entirely in how information is preserved

“parse don’t validate” is a pithy and easy to remember maxim for this preservation.

Because validation is implicitly necessary for parsing to a representation which captures your invariants anyway, by banning validation as a separate concept you ensure sole validation doesn’t get reintroduced, because any validation step outside of a wider parsing process is considered incorrect.