It is just a song. Speculation is fun but it leads nowhere.
As Malcolm and others have pointed out, the song appeared (1666) long after the Gnostics, Cathars, Templars and the 'kings of Cologne' legend. Might as well trace it to some Druidic folk memory.

There are variations, folk-generated, by people who had to have the Bible read and interpreted to them. They added their own contributions to the stories.