Of course he will, but it could still have happened. Those cases you're not ignoring, but you're adding them to those where he needs to switch.
a) 1/3: real bomb selected, fake dropped -> keep switch where it is
b) 1/3: fake bomb selected, real dropped -> rtb
c) 1/3: fake bomb selected, fake dropped -> switch to other bomb
Your failure-wizzard is manipulating every case of b) to be a case of c), of course you end up with a) vs. b)+c) then.
Dropping 1 random bomb, keeping the real one by pure luck, can and will result in loss of the real one in the case b), which just isn't the case in the scenario. So you just need to compare a) and c).