I do have a question, though ... and not trying to be insulting. What is the benefit of learning how "not to be moved?"
it's not about not moving. it's about managing force apply to you in static. it's the baby step. next up is to "not be moved" while moving. imagine you try to throw someone and they just move through you and you could not stop them, while they can strike you with so much power, with no wind up, and make bruce lee's one inch punch looked like child play. now, since you mentioned judo, what would you give for the ability to be able to get under anyone without even changing your body position/posture, like squating down below the other person?
it's hard for folks to see the link between IP/IS and the arts that they practice. so most folks said why bother. why would i learn that? why don't i learn to do another hips throw, another uchimata, another koshinage, another shihonage, and so on and so for? why would i learn thing that don't make much sense? it's tedious and boring works. most folks don't have the patient for it.
That's why nowaday, i told folk to not bother with IP/IS. they aren't fit for it.
btw, did i mention, everything about this is a test? like those old time masters, testing to see if the students have the right aptitude to learn the real stuffs or not.