You can view it in here...not sure if that's proof for you (not that it matters but you asked so I attempted to give you)
And he says "it wasn't his intention" so they could have/could not have made it to America. Perhaps he was against them and I read it incorrectly at first. Appearently, the reason the wrote the letter was because John Robison (professor at Edinburgh Universiy and Scottish freemason) wrote to him that he was asked to join the illuminati.
No jackass, he says it wasn't his intention to doubt that they haven't invaded us. Responding to a letter to him where he simply said "no" and then the correspondent asked him to clarify.
Edit: and I looked over that site and didn't find that quote (doesnt give page numbers and none of the letters are this one, a response to G.W. Snyder).
But I did find that he wrote several other letters warning the founding fathers not to let the illuminati into America. So yeah, you're reading this completely wrong and spitting on a guy someone of your mental capacity should be worshipping, but that's none of my concern I suppose, especially as long as you continue to be a comedy goldmine.