With all due respect, Dennis (and my respect for you is considerable), I have watched Ueshiba in randori, in the 1935 Asahi film to be precise, and seen him stand still and have a person bounce off him. Twice. Well, that's not entirely true. He did move forward ever so slightly, just as the uke reached him.
Shioda does the same thing at about the 2:30 mark in this clip
Shioda does the basic movement as O Sensei does in the other "chest push" video. His whole body is already rising in a forward-upward arc at the moment uke makes contact. By the time that uke makes contact the arc is actually vertical, and from there is headed up and backward relative to Shioda.
Since uke was attacking the shoulders, the normal attack is to shove up with the hips from the ground through the soulders and arms to try to teeter Shioda up and out of his center.
Shioda lets him do just that. At contact uke is pushing a now retreating object headed upward, and getting heavier as Shioda's upward momentum dies with gravity. Becasue Shioda has overcentered uke's arms upward, uke's shoulders have popped out of line to apply the push. Shioda also allows himself to be acelerated backwards, just like uke wanted. In combination this stops the motion of uke's shoulders forward -- but not the angular momentum of his already committed torso rotation. It has to go somewhere. His hips continue coming forward but with even greater acceleration in the established rising arc as all the angular momentum he was trying to apply from the shoulders is transfered to uke's hips in the same direction of rotation.
Shioda does exactly the same thing when attacked from behind immediately after the first one at 2:33.