Yeh, saw that and I suppose at some point in my life I will have been told that, however it is like saying Hans Christian Anderson did not write his tales or even Aesop writing his.
RB said he is first to write it down so lends his name to it and of course he recognised a good bit of work when he came across it. At least it has been proven that it is not written by an Englishman.
Still sung, in many languages, over the world, except in English!