> The author fixes the problem by just increasing the threshold for changed pixels needed to receive a reward. This is a good idea, but might run into issues in later parts of the game where the screen is mostly animated water.

They could've also implemented this by reading the game memory. Pokemon Red internally keeps track of which areas the player has visited before.

I think the reason they opted for the visual approach is because they didn't want to give the AI any knowledge that isn't readily accessible to the player. Or they simply wanted more granularity than what the game keeps track of.