That actually makes the observation
more interesting in some ways--none of the YC examples were "founded" by YC either.
One could argue that Musk has acted like a very aggressive version of an angel or VC--finds promising young startups in growth industries, provides them with capital (or, in Musk's case, some capital and lots of sweat equity, to the point of getting himself listed as the CEO and over-shadowing the original founders), and helps them to blossom into a precious unicorn.
Either way, it's interesting to think of Musk's personal endeavours in comparison with a group like YC. I don't really know what conclusions I would draw though.