This is interesting in and of itself. I think it would be interesting to have a linguistic conversation surrounding it. This is akin to memorizing gibberish (in this case, words without meaning). Which is more difficult than it sounds!
I'm amazed that he also (intuitively) figured out "the French sound" (the linguistic term escapes me) well enough to be able to call out words for not being words. I wouldn't be surprised if many French speakers tried to sneak some high scoring words past him, knowing he can't speak French only to be flabbergasted when he called their bluff.