The mound is more stressful due to the "falling downhill" increases the angles of all the joints and increases all of the muscle forces to slow the body down.
It increases the acceleration but also increases the deceleration forces needed, is another way to look at it.
There really isn't a significance for the position player ti throw on flat ground. I think he was just including him with Lynn to say he is throwing. Which is significant as throwing places a great deal if stress on the hamstrings.