I guess there is quite a paradox, that non-native English speakers know some grammar concepts better than the native ones. I also think that it's an extreme example and somewhat I refuse to believe that college students do not know the difference.

I also was told that non-native speakers can better distinguish between 'well' and 'good', and the usual suspects 'you', 'your', 'you're'. The latter is so basic that I am bewildered when native speakers cannot understand the difference.