Inverted, his belief that Nostradamus' predictions will be correct is justified so long as you believe he could see the future. A decent brown townogy of this would be a slot machine. Say a person wins the slots on his first try. I'm guessing anyone would say "oh, you're lucky, whatever." But, say he wins 100 times in a row. You would be the person who says "He still will probably not win the 101st roll. The machine isn't broken, he's just been lucky so far, but he still has just as much of a chance to lose as he did the 1st time he tried the slots." Riot would say "No, no way will he lose. The machine is rigged, he is definitely going to win again."
Now, please note I am not saying you are wrong. I am saying you calling him an idiot is unjustified. How can you know if he will win the next one? Well, there are 2 ways. Either let him try one more time and see what happens, or inspect the machine. If it's rigged he will win, if it isn't then it is all up to chance. Since we currently have no way to "inspect the machine," this is all guesswork until the time of his predictions actually come.
Please note that I have only heard small tidbits about Nostradamus, and that I actually do not know much about his predictions. This whole scenario relies wholly on all of his predictions thus far having come true. If any one of his predictions have not come true the whole scenario is invalid and you are entirely just when you call Riot an idiot.