Fair enough. I'll bow to your greater knowledge... but so should a lot of others

. If it is a myth (and I'm not saying it is or isn't) it's a well established myth. For example, I have heard (second hand, so as reliable as you want to make it) that one of the north American cultures had the Europeans play the 'divinity' angle on them, which to the surprise of the Europeans didn't work and they got their arses kicked. Now that wasn't read anywhere with any ability to reference, so it quite easily could have been a crock, but it IS something I heard... which means that a lot of people believe the myth.
I don't think it changes too much though. A mesoAmerica with decent technology gunpowder would have a tremendous advantage. The point you made earlier about the Europeans losing the first engagement and Cortez not being deterred, well, wouldn't it also be quite reasonable to assume that he wasn't deterred because he had the advantage of ships that couldn't be touched, so could land where he wanted - "hit 'em where they aint" so to speak - whereas something like
THIS would certainly mean that the Europeans could be hit.
I'm not saying that the New Worlders would have a series of forts that could fight of Overlord. I'm saying that it would change things pretty dramatically.