He memorized the series of valid letters. That's it. He still has no idea what "ongle" is, just that it's a perfectly valid French scrabble word. Certainly just knowing all the legal sequences of letters does not buy you much.

If you study a language more distant from English than French, you'd be surprised at how even knowing all the meanings can still leave you pretty baffled at the meaning of a sentence.

You do have to get farther away from English than French though; I can still half-read French off of a 4-year "not all that great" study in high school, and that's more a testament to how knowing enough English to recall Latin roots we don't use in our main vocabulary and some of the most common French words that are different from English is enough to read an awful lot of French from an English start than any skill of mine. I tried half-a-dozen words in Google Translate to pick my example above before I finally found a word that was either different enough that it wasn't basically the same as it is in English ("ski" -> "ski"), or something with enough Latin roots that English also uses that a strong English speaker would have a pretty decent chance of guessing ("smelly" -> "malodorant").