The real reason an engine with an open exhaust won't idle very well is because fresh air gets sucked back into the exhaust port and leans the fuel air mixture. With a muffler, only air that has the oxygen burned out of it reenters the cylinder and the air fuel mixture gets diluted with exhaust but does not go lean.

Spark ignition engines won't idle either without at least some sort of header pipe on the exhaust port so there goes the "you need backpressure to keep the glow plug hot" theory. Engines that only run at full throttle (control line engines) do just fine with an open exhaust. Rotary radial engines used in a lot of full scale WWI aircraft also would not throttle down for the same reason so most of them used a kill switch to run the engine in bursts when waiting to take off or during landing.