I am writing this on my own experience. I have used only two bookies for arbing , book A and B.
Book A is sharper and my balance there was always negative ( lets say -2k)
Book B is softer and my balance there was positive ( lets say 10k )
If i had bet only the side at B book which is the soft one , my profit would be +10k . Actually it's 8k ,because i have lost 2k at Book A which is the sharpest one.
This is the point that Sawyer is claiming.
Now back to his example.If you arb between Pinnacle and a local book ,you have two choices :
A) to bet both sides . Long term balances will be ---> Pinnacle negative ( -2k) , Local pozitive (+10k) , so the profit is +8k
B) to bet only at local ,cuz you know that you will lose at Pinnacle in long term. Profit 10K