According to Joseph Henry Jackson's excellent introduction to the University of Oklahoma Press edition of John Rollin Ridge's ("Yellow Bird") The Life and Adventures of Joaquin Murieta The Celebrated California Bandit (Norman, Oklahoma: 1955) the buried treasure yarn was first published in Sunset Magazine in 1925.
I very much doubt there's any more truth to it then there is in any other part of the Murieta legend. It's fiction from soda to hock.
Good luck to all,
The Old Bookaroo