If we're using "doesn't matter if it's low or avoidable if it can happen", then it is comparable.
A 1/9 gamble to convert a throw at low % is silly too but its not comparable to a gigantic RNG instakill hitbox that the player can choose to throw out almost instantaneously (that also hits under the edge), cancel, or swap to another move. "but what about this other thing that has rng" is just a slippery slope argument.
Its not even just about Thwack either, that's just one of many questionable scenarios that arise.
The other reason its not comparable is because it scales by %.
if this is accurate then:
Hit/Miss - Chance (dmg %)
190/200 - 95% chance (150%)
140/200 - 70% chance (130%)
108/200 - 54% chance (100%)
72/200 - 36% chance (50%)
meaning its a huge chance of killing at even low % compared to game and watch being a consistent 1/9 chance accross the board.
So tell me how that's comparable?