How is getting an error array not making invalid input unrepresentable.

You either get the correctly parsed data or you get an error array. The incorrect input was never represented in code, vs a 0 value being returned or even worse random gibberish.

A trivial example: 1/0 should return DivisionByZero not 0 or infinity or NaN or whatever else. You can then decide in your UI whether that is a case you want to handle as an error or as an edge case but the parser knows that is not possible to represent.