It also depends on social circles. If you hang around with a bunch of eton/oxbridge old boys, even as a non native english speaker closing in on my 15th year in London, I struggle to understand many of the expressions. But I don’t when watching the BBC. Also as the article mentions there are many terms that are very infrequent, and even as a native French speaker, I had to look up a few cathedral terms in the dictionary.

But I kind of disagree with your “God have mercy on my soul” statement. French people are extremely proud (and being a French native, rightly so!). There is nothing they appreciate more than the world looking at them. When the international press makes their cover on France, it usually gets reported with screenshots in French media, something I have not seen in any other large country. For the same reason, most French people tend to feel flattered when someone makes an effort to speak their language. Though there are enough ancient and illogical grammatical rules to ensure a non native always gets spotted after only a few sentences.