I actually agree with Lucas on this one. There's a strong supernatural vibe throughout all the Indy films. Even though the displays of the supernatural are not many, the belief in them and the quest for the power granted by these objects are omnipresent in the villain's motives. I get what he saying: these are supernatural mysteries (well, actually "quests" or something would be more appropriate) in the garb of action-adventure movies. The same way the Matrix films are philosophy discourses in the garb of technopunk action films.