You read a published book, e.g. Robert Jordan, there are a few errors every chapter. And that probably after its been edited ten times between author and publisher.
A writer has a responsibility to be good at english (or the language the MS is in), but that's going too far.
What they are saying is they only want someone of great caliber, don't even bother everyone else. A good way to save time, sure. A good way to miss some rough gems that only needed a little polishing, definitely.