This is not a rule. A lot of the players don't use much math--basic pot odds. They develop a feel for when they're making the right play, and openly admit to being "feel" players.

I do believe they would be very good at doing on-the-spot calculations in their head, they're often just not very trained in math.

Anyhow, surprisingly little in poker is math-dependent, particularly in normal ten-handed games. The cognitive load is mostly about factoring in thousands of variables, past hands, and potential hand combinations of your opponents.