It's actually not. The reason he mentioned what's going on on the inside is not what you think is going on by looking from the outside.
Sit in that same position, and then do a lift part way. At some point, you'll find a "spring" balance, where you can autonomously drum your leg. The muscle that is doing that triggered flex is the only muscle you are focusing on with the Soleus Pushup. Except for the SP, you are triggering it with a pause instead of letting your leg hammer. That rest will allow you to do the move indefinitely without muscle fatigue in that muscle only. It's a bit like your heart beat. Other than the pause, the distance you aim upward for, and the catch (the small bounce right before your heal falls to the ground), the move is way more like this leg drumming than any calf raise, but slower and with rest strokes.
Incidentally, you can google the paper, which includes graphs of the muscle use. The two main muscles engaged for calf raises are only nominally activated (because it's pretty hard not to flex with this move) but the Soleus is doing way more of the work.
So visibly it is pretty much identical to a calf raise. But what is driving the motion is totally different. Plus you limit to only 1% of your body weight. That muscle specifically has that spring action the surrounding muscles don't have, and uses nutrients entirely differently in order to be that way.
After a half-hour of doing this, just like cycling, your body enters a metabolic state, which means that muscle is eating up fats and sugars from your blood at a much higher rate than normally. Then you keep going from there because your Soleus is the one demanding the nutrients.