> I've heard that the French are strict about language, but what does "reserved" mean here? What do the French call coke¹?

That's also called "coke", which is why there's a Tintin book called "Coke en Stock". [0]

That said, if you say "coke" in English, almost nobody will think of fuel, and the same is true for French speakers today.

[0] https://www.amazon.com/Aventures-Tintin-Stock-French-Sharks/...