It does look like they have rotary encoders on at least the first 2 joints.
However, I could be wrong. To keep it completely frictionless, they probably need some sort of optical or very low friction contacts prom axis-pivot to the next axis-pivot.
Otherwise, if it is all load cell measured at the moving base, this it is too unbelievable. Because of other videos on Youtube, like the impossible miss basketball net, camera feedback is also possible.