[thombat]

Try to stop assuming that everybody here shares your exact experiences and background. English has acquired many loan words and phrases from French and many people from British and Commonwealth backgrounds learned some French at school, to the extent that Franglais is both a joke in itself and used to make jokes. Saying "I don't speak French" in French is that sort of absurdist/wry joke, not the least because often that is the about only thing a traveller learns of a foreign tongue - how to apologise for not speaking it and to ask whether they speak English.

So although I'm a monoglot Kiwi I'm neither baffled nor insulted by a snippet of French, and although I know little of the history of art the fame of that surrealist painting precedes it. "Ceci n'est pas une pipe" has passed from being a painting to a concise reference to an idea, and like all jargon its succinctness is both useful for those familiar with the field and can be forbidding for those who don't.