I think you're using a different definition of 'force' than I'm used to? I have an engineering background so I'm used to a pretty precise, narrow definition. I.e., he is pushing, so his mass is exerting a force F1. The system is not in motion, so there is an equal opposing force F2 where F2 = -F1, so the net force is 0, since F = ma and you have a mass and are clearly not accelerating... I imagine you're trying to express something more subtle than that with your choice of words (something about what the opposing force actually is? I get that you're saying the muscles are relaxed), but the language you're using isn't matching with my knowledge of the words.
I think so, too. The language not matching between us. This is much easier to talk about in person, but that's not always an option. If you'll note in the vid, when my partner suddenly takes his arm away, I don't move into him. So, I'm not exerting a force back into uke. So, if I'm not equalizing the force that uke is projecting, then something else must be going on, right?
The body is a very complex machine. It's never as easy as basic physics. Think about radar being altered by stealth technology. Rather than sending waves back to be picked up by radar, those waves are trapped and/or redirected and/or lose energy. The body is complex enough that it can be trained to achieve similar results with incoming energy.