They are trying to be funny. In English you can say "Man, this person really knows what they're doing" as a way to say this person is very good at what they do or that they have done or made something very cool or special or interesting or beautiful, etc. The person knows there was not a single person that invented French. They are saying French is really sexy (beautiful) because somebody that was "good at what they do" or skilled made it. Since this is not actually the case, it is a somewhat humorous expression. Ex: You come across a really beautiful garden. Somebody could say "Wow the gardener really knows what they're doing." This would be a roundabout (indirect) way of saying that the garden is really well done or really beautiful.

Basically it is another way of saying that this is a nice expression!