There does not appear to be one in this robot, from what I am able to read about it. I think sometimes people assume there must be something like that to help balance.

This generalizes fixed 2-wheel dynamic balancers which mostly don’t have flywheels for stability either, the focus is on the dynamics of keeping the wheels under the center of gravity, or a bit offset when moving.

The novelty here is about switching between dynamic control policies while keeping them simple.