The information given is that one person steps forward and the other person steps backwards. But, what if that exercise was a push-out type exercise where the person "stepping backwards" wasn't supposed to move unless actually forced to move? So, in essence, there could have been an actual push test exercise to build the skill to withstand pushes. Anyone remember the creation of that exercise?
Perhaps things were altered, changed, modified. But, who remembers?
Actually, the exercise you describe here does exist in Shodokan Aikido, it is called Shotei Awase and is part of the kihon kozo to be practiced at every class. This exercise is designed to develop pushing power by channeling the force of the push from the ground through the body into each hand (specifically the shotei region of the hand). Ones partner provides enough resistance to ensure that you must move him by floating his weight upwards and stepping through him. The arms are held relatively straight so all power must come from movement of the whole body. As ones level of knowledge increases ones partner provides resistance by relaxing his body in the correct manner and timing to avoid his weight being floated (becoming heavier in a sense) and thereby become harder to push.
We have another pushing exercise called Hiriki no Yosei which involves turning and pushing a partner who is providing resistance to your movement as well.
The exercise you saw in the Loi book was Tegatana Awase and it is a precursor to Shotei Awase as far as our basic exercises go.
Just a little tidbit.