The technological problem of power density vs. limbs and the control of the limbs seem fairly separate to me. If we had 10 times more efficient batteries, this would simply run ten times further without any other changes. Unless the bottleneck here is heat dissipation in the motors, but even that would be way easier to fix than our battery problems.

Interesting point, but besides Cheetah being a sexier, more accessible name, the main problem they are trying to solve here is gait and top speed. The efficiency of batteries (heart/lungs) is a ancillary problem they have to solve, but not their main goal.