I don't think Xur is random, but that is an absolutely awful attempt at proving it. There are countless (not actually countless, but I don't feel like dealing with factorials right now) possible arrangements for him to generate, so the probability of him generating any one of them is very low. However, no one ordered arrangement is any less likely than the others, and he would have to generate something. It's like saying the lottery can't be random because somebody won despite the odds against him being astronomical; it doesn't make any sense because by definition somebody has to win.