Hm.. I always thought the deciding factor that determined how much drawbar pull a tractor had was the type of tires and whether they were correct for the pulling job at hand to "hook up" to the surface they were suitable for AND the weight on those tires.
With the more weight on those tires determining if the tires would lose traction with the surface and spin or would deliver all available torque the engine would provide. Of course factoring into this is the tractor in the proper gear for the attempted pull, and hopefully some moron did not put a hydrostatic transmission in too high of a gear range so that the hydro relief valve did not open and thus prevent the maximum power to be delivered to the hydraulic motor. Of course the same could be said if the moron put the gear tractor in too high of a gear and stalled the engine. But again the biggest determining factor is tractor weight, with heavier tractors "outpulling" lighter tractors each and every time as long as their gearing is such to not stall the engines or open the relieve valve and as long as their tires "hook up"
These types of questions have only one answer: "it depends" It is kinda like asking which pickup pulls harder, Chevy, Ford, or Dodge. Well it depends. And it depends on dozens of factors.
Let me ask you a question: What difference in "pulling power" (and first we would have to define what that means to you) would the fact that a hydrostatic transmission loses about 15 percent of the energy going into the hydraulic pump from the engine and between the output shaft of the hydraulic motor IF the hydrostat equipped tractor can and does spin its tires when pulling against an immovable object. So if the gear transmission tractor spins it tires too, what does that tell you? It tells me that more drawbar pull could have been achieved if both tractors had the correct tires fitted for the job and both tractors had sufficient weight added so that the tires would not spin up to the point that the engines stopped or the relief valve opened. But when someone comes along and makes a misleading video to "stir the pot" and stir up FUD what can we say?
Now I suppose It might be possible to cause a hydro transmission tractor to open the relief valve in Low range, I have never seen it and I have owned 3 hydro tractors. Mine have always spun the tires even with loaded tires and a heavy
ballast on the 3pth. Of course I have usually had R4 tires too. Perhaps If the test was in soil and I had R1 tires, perhaps the tires would not have spun and perhaps the relief valve would have opened.