You talk as if raw damage is the only stat that matters.
The Khab has the worst concealment of all the tier X DDs, compounded by the fact that most people use the heal instead of the smoke. Yet it has the best survivability by far, because of its armor and speed. It's armor is just thick enough to resist cruiser and DD caliber HE, but not thick enough to arm AP shells. Combine that with its heal and lack of a citadel, and you've got a DD that is magically more durable than same tier CAs.
Khab can also tank damage consistently by staying at range, using its speed and mobility. Every shot taken at a Khab is a shot not taken at its teammates. The fewer shots your teammates take, the more likely you are to win.
And for all those survivability strengths, Khab doesn't have to trade offensive firepower. Fast rate of fire, good fire chance, it's more than capable of burning any other ship to the waterline. And no other same tier DD stands a chance against Khab in a knife fight, unless the Khab driver has f*cked up royally somehow.
Even cruisers, the supposed predators of DDs, have a difficult time dealing with Khab. AP over penetrates, while HE shatters. Meanwhile, Khab is fast enough to kite it's would be predator while burning it down with HE spam.
Khab isn't OP because it deals more raw damage than any other ship, it's OP because it has so many strengths compared to almost zero weaknesses. And it's raw stats reflect this. It's better than every other Tier X DD in every single statistic. Zao is the only non-CV that has a better survivability rate and K/D ratio. It has a full 2% better win rate than the next best tier X. It has the most average xp per match than every other non-CV. Even it's accuracy is on the high end.
Raw average damage is not the only stat that matters.