You don't know that. You're taking his average return and treating it like an absolute constant return, which it emphatically is not. The period Thorp averaged over was mostly a raging bull market. Every investor in the bulk of the '90s was minting coin.
Princeton-Newport had 227 positive months out of 230 with the largest monthly loss just under 1%. Given all that Thorp is responsible for and achieved I find it very difficult to attribute his success to a bull market.