Let me revise. My numbers were calculated based on 2-card hard hands only, excluding any 7-7 and 8-8 pair hands. Let me calculate these two numbers more accurately by including any-number-card hard teens.
If a stupid player stands on all hard hands, 13, 14, 15, and 16, these mistakes cost him only 1% disadvantage.
If another stupid player hits on all hard hands, 13, 14, 15, and 16, these mistakes cost him only 0.4% disadvantage.
Therefore, a stupid player plays the game at a house edge of 1.4% at most. Still, not very bad at this gambling.