I can't think of an easy way to make string splitting an isomorphism in both directions. You can serialize any list of strings to a comma-separated list, but only if you use escape sequences or encode the length of each item, and then there are some input strings that are errors.
I guess you could define a type as the set of all valid strings in the input language, but that's going out of your way to make sure parsing has to be done more than once.
Parsers that report no errors don't seem desirable anyway; error-checking is usually good.
The title of the post says "don't validate," and so I initially expected it to relate to the mantra of being liberal in what you accept, etc.
So you have to assume there could be missing words, since that’s how headlines work.
In this case there is a dropped “just”:
Don’t (just) validate: parse!
If the headline was “Parse, never validate” then I would agree with your reading. But “don’t” is pretty lightweight. I’m having a hard time thinking of a lighter discouragement word. Given that, I think it’s unfair to assume the strongest discouragement is implied.
Partiality is what you get when you choose you only accept one branch of a disjunctive structure.