The point is that people, by the nature of their geometry, don't have inherent static postural stability. A person who really accomplished your 'rock' would not be hard to move at all - any more than a baseless human statue would be difficult to tip over if you pushed in the right direction(s)
I don't this is the kind of "rock" Ledyard Sensei is talking about. Sure, if you just tighten up your body structure by "locking" up the skeletal system with muscular tension, then you become a tottering-easily unbalanced-statue. This is what an untrained person would do, but then again, I don't think anybody is talking about unbalancing or moving untrained individuals here. If your weight is sunk down in a more relaxed and dynamic way, meaning being responsive to pushes/pulls/application of energy, and running that into your feet, then you become very immovable. But, like Ledyard said, this requires a complete pulling inward of your own energy, nothing flowing outward, only toward the ground, and no response to your partner's energy except toward you own structure.
Just to be fair, and be open about my bias:
Oh yeah, I absolutely loved the Endo Shihan video. That is what I am aiming for, so put me on that side of the fence...I trained the "other way" for years; left it when I experienced Saotome Sensei and his students.