It's the direct drive motors that make it work. As someone from GE said a century ago, "You can't strip the teeth of a magnetic field". Direct drive robot arms have been built; Adept SCARA arms were direct drive. But they used to be too heavy for mobile robots.

What makes direct drive motors work are today's really good power FETs. You can do things with FETs, capacitors, and switching power supplies to get whatever voltage and current you need, at least briefly. Motors can be overloaded by huge amounts if you monitor the temperature, and you can build up a charge in supercapacitors for the next time you need to hit the motor really hard. Putting a computer in charge of a power supply is normal today, and may even reduce the parts count.

This, and Schaft, probably spells the end of Boston Dynamics' hydraulic monsters. That approach is just too bulky for anything below mule size.