A more likely solution was that the oil in there was SOOOO spent, so thick with crud, and the additives were so worn out, that the trans fluid he put in actually WAS a far better cleaner/lubricator than the oil that was in it, and thus DID manage to do something that the oil that was in there couldn't.
Think about it, if the oil that was in there was so used up that the TBN was at or near zero, and the TAN was up there, it wouldn't have been able to do any cleaning, and would actually have been causing sludge.. but fresh clean ATF would actually have a positive TBN and SOME active detergents.. plus it would have been CLEAN and capable of actually picking up existing sludge deposits.
... Just a thought.