He concludes that because he assumes what the judge said is true. If we assume that, we must reject the scenario where he gets executed without surprise.
Actually, you're right. However, the paradox arises no matter what the prisoner concludes. If he concludes that the hanging won't be a surprise, he will be surprised by the outcome (the knock on Wednesday).
Let me say it again in a different way. My resolution of the paradox is that the prisoner incorrectly negates "x will happen" as "x won't happen" instead of negating "x definitely will happen" as "x might not happen". Thus,the prisoner cannot conclude that the hanging won't be a surprise. The prisoner can only conclude that the hanging might not be a surprise.
> he assumes what the judge said is true
The judge says he will be hanged. Your comments are inconsistent.