Seems like an excellent definition, mainly because I have no idea how to measure it otherwise. "Complexity is the time needed to solve a problem", why not.
Well, there are a lot of shortcuts built into engines like stockfish that don't perfectly mirror how beginners vs advanced players think.
For example, certain types of piece forks are easy for newer players to spot (knight forking a king+rook on the back rank) and others are harder (bishop forking a king and a pawn in the late game when nobody is on the back rank), but stockfish is going to find both just as easily.
So if you want to get deep into complexity you do need a model of different players and what types of advantages are easy to see for them.
Well, there are a lot of shortcuts built into engines like stockfish that don't perfectly mirror how beginners vs advanced players think.
For example, certain types of piece forks are easy for newer players to spot (knight forking a king+rook on the back rank) and others are harder (bishop forking a king and a pawn in the late game when nobody is on the back rank), but stockfish is going to find both just as easily.
So if you want to get deep into complexity you do need a model of different players and what types of advantages are easy to see for them.