He placed a bet on another player (John Wall) to score more points because the absence of James Harden means that player will play more minutes and will take more shots because Harden takes 24 shots per game. So those 24 shots will be distributed among other players, but since some players have more "shooting privileges" than other that means that player will take bigger % of those 24 shots.

Let's take for example game between Houston and Sacramento that was played on December 31st.
Harden played 38:18 minutes and scored 33 points from 24 field goal attempts and 11 free throw attempts.
John Wall played 36:34 minutes and scored 22 points from 20 field goal attempts and 4 FTA (free throw attempts).

Houston and Sacramento then played on January 2nd (the game in which Harden did not play)
and John Wall played 37:14 minutes and scored 28 points from 22 field goal attempts and 6 FTA (free throw attempts).