Well, not necessarily. More like a 50/50 chance for either 'mon.
Now, obviously Gengar is destroyed on the switch if it's hit by Crunch, but let's just assume the Gengar player predicted well and got it in unscathed. It's immune to three of Kang's attacks on the switch-in, after all. From there, it becomes a guessing game. If the Kang player goes for Sucker Punch (turns out Focus Blast is a guaranteed OHKO from Timid Life Orb Gengar w/ SR, 81.3% chance w/o), either the Gengar dies or it gets its Sub up and proceeds to force Kang to switch out. On the other hand, if the Kang player Crunches then it either risks the KO from Focus Blast or takes down a Sub.
I agree that it's not the best answer to Kang as it relies almost entirely on guesswork, but saying that it can't work isn't technically true.