Please note that I'm making no assumptions about the history of this particular song, and have no opinion as to its age or provenance (except with reference to the bizarre theory I mentioned earlier)!  I made a general observation about the tendency people often have to insist that a song must be very old, when they actually have no evidence.  They may be right, they may be wrong; but without documentation it can only be a guess, and should not be stated as "fact"; merely (sometimes) educated guesswork.  If there were a reported attribution of Shenandoah to a supposed composer, it would be quite well known about, I should think.

Malcolm