Musk is referring to an urban legend — apparently spread by cheese-eating, upper-class French surrender monkeys — that Catherine was crushed to death while trying to make love to a stallion. (In reality, she died in bed of a stroke at age 67.)
Although his choice of Twitter topics is odd, Musk’s selection of reading material is not. Massie is the Pulitzer Prize–winning author of Peter the Great, Nicholas and Alexandra, and The Romanovs.
Oh, I see he also made some comments about the Chinese and the state of rocketry which seem reasonable enough and I’m sure have no connection to his other topic.