Martin, I agree. My bewilderment has less to do w/questions of his scholarship than I made it seem. I stand w/you on your comment:

I suspect his "Originally it was a song, not a shanty..." is wishful thinking. He thinks "shanty" implies "sea" and "Missouri" implies "not-sea" so.... . The difficulty is that it implies adoption of a shanty at a very late stage??

and ask the same thing.