I don't recall HashFast ever saying they'd get 400GH with 40 watts.
I guess that's a typo and should've been 400 watts.
Did the creator of this bet just make up something impossible to ensure Hashfast can't win?!?
It's not about HashFash winning, it's about the one who made the bet winning. So maybe it wasn't a typo and he bets on people not reading carefully enough and them thinking "sure they'll do better than 400W for 400GH/s" and bet against him. I'm not sure how strict these bets are judged.