I would consider the fact that this is such a central aspect to Iwama style aikido to be evidence that he did. Further whenever I was able to train with Kurita Minouru Sensei (one of the last uchideshi of OSensei, who spent a great deal of time with him at the very end of his life) he always emphasized this kind of training at some point in the seminar. He explained it as a stage one needs to go through.
I was trained this way as well by Saotome Sensei who was with the Founder for 15 years... I have only come to realize recently that this was not ideal in many respects.
Yes, you as the nage should be able to handle the strongest grab with ease. That's what you are training to be able to do. But the fact is that this kind of grab is not martial. it has nothing to do with actual martial application. The point of grabbing is to neutralize one limb and effect the structure of the opponent so that his other possible strikes are limited or eliminated. That might mean he falls or it might simply mean that you have set your own atemi and placed him in a spot at which he can't protect against it.
There simply is no function in grabbing someone and hunkering down so that they can't move. You need to effect their structure. Proper grabbing is the same as proper receiving. It's just a connection point. The grab should be firm enough in the grip that it doesn't come off easily, and the rest of the arm is relaxed (like a newborn baby which can hold its body weight up with its grip and there isn't another tight muscle in its body. At the instant of contact, direction is imparted to the defender's structure by moving the hips and dropping the weight or shifting the body weight forward... The way that you do this is exactly the same way you receive the grab to do technique.
I have observed that many of my fellows misunderstood the purpose of strong grabs in training as meaning that this was the way to really attack as opposed to it being strictly a training exercise in which one person acts in an essentially unreal manner as a test of the other's understanding.
Endo Sensei has been trying to get everyone to understand that training is the process of removing tension from the body and mind. Since 50% of ones training interaction is is in the role of uke, if you are tight and resistant as uke and then think you are going to be relaxed and sensitive as nage you are crazy. This is improper training even though many of our teachers allowed us to train this way for decades. If you don't let go of it, you won't get where you need to go.
The grab in static technique isn't about locking into the other guys structure, that actually makes it easier to do something to him as Chuck Clark Sensei pointed out. When I grab my partner I am owning the space right up to the point of contact with my structure and touching the partner's center with my extension. If he collapses in trying to get away from my structure I simply follow him in and eventually own all of him with my structure. If my partner pushes into my structure in any way, I am simply immovable. I did not grab with the intention of stopping my partner I grabbed to establish a direct connection to his center. Static practice takes place when I choose not to initiate a technique at that point but allow my partner to experiment with how he can use that connection to get me moving.
If it were a martial interaction, if he didn't have me at the instant of contact, I would have him. That's a more advanced practice and one that people try to jump to without fully understanding that basic static form. As Chuck said, none of this is actually rigid or tight. It is alive and sensitive. If you are tight you are feeling you, if you are relaxed you are feeling the other guy.
There should be no aspect of training that imprints tension. It's ok once in a while to have someone stupidly resist but its just as a check that you know what you think you know. Daily training is about programing your mind and body to think that relaxing will make it safe rather than tensing. If half the time you are tense, your poor body simply gets confused about what it should do.
Let me reiterate... this does not mean that I am falling down for my partner. I can grab my partner with a grab that is quite gentle and he will be unable to move me unless he does the the technique properly. In fact i can place my grab and then actually open my hand but keep contact and still stop my partner unless he does the technique properly. It ain't the power in the hand that stops him anyway.