>It seems like an “easier” planet would require less intelligence and a “harder” planet would require more.

I'm not sure that would be the case if the Red Queen hypothesis is true. To bring up gaming nomenclature you're talking about player versus environment (PVE). In an environment that is easy you would expect everything to turn to biomass rather quickly, if there was some amount of different lifeforms so you didn't immediately end up with a monoculture the game would change from PVE to PVP. You don't have to worry about the environment, you have to worry about every other lifeform there. We see this a lot on Earth. Spines, poison, venom, camouflage, teeth, claws, they for both attack and protection in the other players of the life game.

In my eyes it would require far more intelligence on the easy planet in this case.