One of the requirement listed to apply via the author's company is native fluency in English. How do they evaluate that? If it's some test, I am not aware of any test which measures this. If it depends on where you are born? Is that kind of stratification legal?
Of course it’s legal and there are numerous language proficiency tests. I don’t see what the problem with that could possibly be, just like requiring a candidate to have a certain amount of proficiency in Python.
If they mean "as fluent in English as a native speaker", then that's arguably much too high a standard.