It's not a Goethe moment for Mr. M. There is a certain flexibility about the term cul depending on how it's used. It means 'end' as in cul-de-sac and it could be used in expressions to mean cunt - just not as often as con.
By the way, my French isn't especially good, I just have plenty of French nephews and nieces so my exposure to swearing in French is quite high.
I was reasonably confident of my ground, cuntwise, largely as a consequence of a charismatic, liberal and rugby-addicted French teacher in secondary school. He knew and was happy to share with his students a wide vocabulary of pardon-my-French epithets that he had picked up on the touchline.
"We all have flaws, and mine is being wicked." James Thurber, The Thirteen Clocks 1951