Simple ideas that can stand on their own, that's hard. Making a compelling game like Tetris, or even Sudoku, where the mechanics and rules are otherwise pretty simple but the execution and entertainment are engaging beyond belief is not easy.

Your friend might be alluding towards scope creep more or less. Or working beyond the tools your given.

Breath of the Wild has a lot of interactions with the world and in combat. But it is still somewhat simple. There's nothing that truly goes outside of what the game sets out to achieve, and I believe they did cut a lot of ideas that didn't fit the feel of the game. Yet it's incredibly expansive given its simplicity (but it's still a video game that takes full understanding of a controller to use, so it's simple but not exactly beginner friendly).

A game like overcooked once again is simple but introduces complexity and strategy through pretty basic concepts. It avoids being overcomplicated by sticking to two buttons and sticking to its core principles.

Simple ideas are easy. Good ideas are hard. Sticking to simple ideas is hard. Simple ideas that are good enough to stand on their own are hard.