I don't think SpaceX ever made that accusation. At the time (2016) Musk said the cause of failure was unknown. He was asked directly about sabotage on Twitter and replied that they didn't rule that out. As far as I can tell, the first time anyone at SpaceX talked about the rifle hypothesis was Musk simply saying SpaceX looked into it (the implication being it wasn't sabotage).
WaPro: http://archive.is/j8zjc