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.
Of course feel players are doing EV analysis, but that's 99% about estimating hand ranges properly. The math is certainly the most simplistic part of the analysis.
Could you outline the process or point to a place to read up on it WRT poker?
There used to be prominent winning players who were unaware of the math behind the game, but the theory nowadays is they mostly just lucked into naturally playing in a style that happened to be better than the field during the period of time they played (i.e., years ago back when the game was less sophisticated).
Nowadays, those players are often the weak players who the players more skilled at theory can comfortably beat.
There's an eerie parallel to the early stock traders here.