what he means is if its tuned and maxed out at 20psi, if the car overboosts or you turn the boost up the state of tune will not change as the ecu does not know the boost level has changed so it could cause detonation or a too lean of a mixture for the extra boost.
it works the other way as well, if the afm max's out at 17 psi and you tuned the car to 23psi, then you decide to turn the boost down to 17psi, then it will run too rich and you won't be getting the right power / mixtures for that boost level
So its kinda ok but not recomended to tune the car with a maxxed out AFM, if you dont intend on changing the boost at all then you can possibly get away with it if you have a decent boost control that will never fail and overboost