I then went off to learn more about the chanson

What you should have learned is that (like "lied" in German, "song" in English or "canto" in Italian) it has no very definite meaning, and people have applied the word to unrecognizably different things over the centuries.

Brel was not doing anything like what a Renaissance composer did.

There are languages that have hard and fixed terms for different kinds of vocal music - Turkish is quite good at that. Western European languages prefer to leave you guessing.