edit, I didn't look the vids before my answer, but I keep it as is.
It's just a direction change, no?
No. That would be the case if you are on the ground trying to lift a load with just a rope and an hanging pulley. Your force/weight counterbalances the load. 1:1
It's different in the tree, as
you are the load. It's called a closed system. Basically, pretending that we consider only the tensioned part of the rope (cutting out the slack part), both ends of the rope are holding you. So each end sustains half your weight. What you do is shortening the rope on one side, but you are still hanging on the both ends of the rope, either pulling with your arms or staying as is on the prussik/ascender. Half of you counterbalances half of you, or half of you lifts the other half. So it's 1 (you) lifted by 0.5, aka 2:1 to simplify. Actually you needs a good amount more than the half due to the friction of the rope on the bark, but adding a pulley brings it closer to the physical principle.
Ditch the body trusting. It uses only one ascender point on the rope. You have to jerk up yourself to reduce most of your weight (like jumping with no bearing point under your feet) and count on your arm's speed to take out the slack in the quarter of a second before your weight is fully seated again. Adding just a foot ascender is life changing (second choice with an hand ascender, adding both tends toward a pleasure !). That brings a second ascender point alternating with the first one. Exactly like a farm jack with it's two pins.
Ddrt seems the less tiring because it needs the less force to lift yourself. A foot ascender and a pulley at the anchor make it "almost" easy. Actually, it's very good for the short distances (small trees and local moves in the canopy for the bigger ones). Beginning the training of your body and enhancing its strength is less a hard step with ddrt. What's hard on the body is the number of movements involved in the long ascents: like you just climb two trees instead of one.
The energy loss in your muscles is the main point. If you can reduce the movements, you reduce the loss, more efficient and less tiring.
Srt needs to lift your full weight, so it's hard (and asks for more working out), but you do it only half as much as ddrt. So, globally, you save on the energy spent.