No matter how fluent, there would be noticeable differences. it's best for writers to write their native language .
Not to bash on your parade, but you're wrong. And I'm not speaking about myself, either. Even though I wrote, and still write for companies that are based in the US, that service US clients (mostly), and are not anywhere near the "shady" bunch that you see students complaining about (I avoid those like the plague as should every reputable writer).
In general, there are individuals who can reproduce the spoken, and written, language that is not their native with complete accuracy. Just look at some of the writers who published in the English language whilst being foreign born, or many others who write, or speak, perfectly, and work in some aspect of publishing or related fields. Or those who published in French, Italian, or any other number of languages that were not their own.
The problem is, for each one of those, you get a thousand completely illiterate hacks, but that's to be expected I assume, even if it's a shame. Point being, what you said is not even remotely close to a truth, as you say. However, it is mostly true in this particular industry which is why it is such a dastardly affair for naive students.