Makes sense to me. I cannot see why the parallel sided machine screw would NOT have greater holding capacity. The only way the tapered screw would have close to equal holding would be if the tapered screw were put into a tapered hole that allowed the threads somewhat near 100% grip, without tightening to the point that wedging (or splitting) occurred.
With a tapered hole and screw, when it is being pulled out by a force, once it moves any at all outward, the only thread having full engagement is the outermost one (largest diameter thread). However, with a parallel sided screw all threads are holding an equal amount when an outward force is applied, even if it moves.
Another way of saying it is, once a tapered screw is moved outwards of it's "seat", the only wood that has 100% holding capacity is that wood above the top thread. When a parallel sided screw is pulled outward, all the wood between all the threads has to shear for the screw to break free.
MY opinion only, SRH
