Here's my logic / assumptions: How does the robot "know" (have sufficient model information to predict the response) that the board will hold it when it does a vault? It doesn't -- it is told to vault by placing one limb on top. As we've seen with self-driving cars, where the sensing is 99% of the problem, that's the barrier between a closed set and the real world.
Don't get me wrong. The fact that Atlas / Boston Dynamics is in a state where the sensing is _mostly_ what remains a problem is astounding.