This guy is really dense af. You realize Chinjao is bragging about the hardness of his Haki? It's not just about learning techniques. It's about the raw power and CoA behind the attacks.
It's a "one shot" because it's an all or nothing clash. If Garp was overpowered there, he'd be one shot too.
But Garp is an idiot, for destroying mountains in advance for a "fodder" right? What a fucking moron you are.