I think I know why. In French "la notion d'avocat est inconnue" can be about a country where Law barely exist and the police or the army can arrest people and do whatever they like with their prisoners. See what I mean?
"the lawyer's notion is unknown" translates in "la notion de l'avocat est inconnue" (meaning: something he said).
I agree and I would also pick "concept" in French for a clearer meaning, because "notion" is generally "de quelque chose" and not "de quelqu'un". In the current sentence, I would change the French to: "la notion de défense est inconnue" or "le concept d'avocat est inconnu".