The reason they said the shorter action was less stiff was the fact that there is less metal in the back of the action in the Model Seven than there is in the comparable SA Model 700 action, and therefore less strength, due to less mass. So, not quite equal in all dimensions.
Otherwise, yes, a short action would normally be stiffer than a long action of same design.
Here's a link to view:
https://rifleshooter.com/2015/05/remington-model-seven-and-700-comparison/
Note: a correction to my first post; I stated that there is one rear action screw, but article shows that memory was off. There are two, just very closely spaced together compared to the Model 700 rear mounting holes.