I don't see how. For it to be so, then some things have to be assumed without evidence.
- That the letter was indeed written shortly after the assassination
- That Gordon hadn't previously discussed the events of the day with his wife in an earlier letter or phone call
- that not enough time had elapsed for him to have gotten to Seattle before writing the letter
One weird thing is that I know the World's Fair was in NYC in 1963. But I looked it up, and there was a "Century 21 Exposition" (also known as the Seattle World's Fair). The problem is that it took place between April 21 and October 21, 1962.
I thought circumstantial evidence was "assumed evidence".
I'm assuming the letter is indeed from Gordon Arnold and that it was written by him. I'm assuming the letter isn't a forgery. I'm assuming the site I saw it on isn't fake.
I'm assuming the Sixth Floor Museum knew what it was talking about about.
I'm assuming a really large amount of things, there can't be any denying that.