I remember reading about this 20 years ago in that "Hammer of the Gods" book.

IIRC, basically, Robert Johnson didn't write that stuff either. It was all based on spirituals and slave songs, right? All LZ did was make these heavy and loud arrangements of them utilizing new instrument and recording tech to make the stoner kids go crazy.

If there should have been royalties paid, I have no idea who shoulda got 'em.