JIM ,Presumably Mark Anderson learned it from someone else.
with the greatest respect it could be argued from your argument,that Mark Anderson was not the original source anymore than Carthy,but he was the last known source.
somebody must have written the song originally,yes it may possibly have been added to,all traditional material is author unknown,but is originally someones composition.
Mark Anderson did not write the song,he was part of a chain,just as Carthy and S and G are.that does not mean I dont appreciate Mark Andersons role,as a song carrier.
Dick Miles