I mean we don't know how he did it. It's implied you need to eat a part of a DF user (likely their heart) to take their DF from them before it respawns.
Eh, even if you're right I don't see a reason to care. He can defeat Imu, it's not like he'll actually be the permanent king of the world as he's destined to die to Luffy (and maybe a Marine like Koby or Smoker to mirror God Valley)