They say that both French and German are "closest" to English but still different languages
Who are they? Because they are wrong. The closest step cousin to English is Frisian. Dutch is closer to English than French, I'd argue.
Novel ≠ novel. It would have to be contemporary and not written in overly complicated language. Novels are mostly not written for learners to read. Even people who are fluent will miss stuff because culture doesn't come in a dictionary. So can they understand everything at that level? Nope. Can they understand enough? Probably. Arguably more so in German because it doesn't fuck with a version of the past tense that only shows up in literature.
That's not a great statistic. Future diplomats will have college degrees. They will have pre-existing language knowledge. That's too narrow a sample if you ask me.
And they wouldn't teach them Frisian anyway because it's a minority language spoken on the mainland North Sea coast from The Netherlands to Denmark.