Tyson's words: "I'm saying, the day we learn that it is true".
Do you agree that by 'that' he meant the alien 'simulation' that was the hypothesis of the debate?
There are only a limited number of things that 'learning that it is true' could mean in this context. It would have to be something like the aliens showing up and telling us, we accidentally discover the alien computer running the simulation, or something similar. Do you have an alternative explanation for what that meant?
And clearly, he is then saying that he will not be surprised: "I will be the only one in the room saying, I'm not surprised."
How could he not be surprised unless he is not fully convinced that we arose by natural processes and evolution?