Leaning for density altitude is appropriate. The pilot experienced a rough engine, and returned for maintenance. He performed a post maintenance runup to determine that the problem had been solved before taking on jumpers. He did not perform another runup prior to that departure, but that may be excusable as he had just done one.
It's worth noting that the use of the word throttle and mixture is interchanged in this account, and the information is only preliminary. Inaccuracies already exist that are obvious, and other may exist that are less apparent.
Strictly by way of observation, generally leaning isn't accomplished per the manufacturers recommendations above 75% power. This normally won't take place until below about 25" of manifod pressure. At field elevation, the airplane would have been capable of developing about 27.5 to 28" of manifold pressure. Excessive leaning can cause detonation, and can cause rapid temperature rises which may result in burned valves and a melted piston head.
The power loss sounds indicative of a lean (or overly rich) mixture, but leads me to wonder about an induction leak adjacent to a cylinder or the crossover tube. However, this would normally lead to a higher manifold pressure as pressure differential is bypassing the throttle plate. If the mixture was sufficiently lean owing to an induction leak coupled with manual leaning, reduced power would be available, and at reduced RPM, a reduced manifold pressure would be evident, which was the case here.
The actual cause has yet to be determined, obviously. Second guessing won't solve the matter. What is obvious is that the pilot managed to salvage a bad situation and escaped without injuries.
Last year some respected associates were killed during a jump opeation when the airplane they were riding in crashed. The owner of the drop zone was among them. A short time before that occured, he had been heard to say that he knew he would never die skydiving; his training and his equipment was too good. He said that if he were killed, it would be by the airplane, and he was right. Most jumpers are comfortable when they have enough altitude to get out, as they trust themselves and their parachute much more than the airplane. Events such as this explain why.
It must be a terrible feeling to know you have two good parachutes, but are too low to get out and use them.
