That's actually not necessarily the case, what matters the most is FPI (Fins Per Inch), on said radiator as I have a 60mm thick rad with a low 8 FPI fin count, and it does not require a push/pull fan configuration, so there are extenuating circumstances to additionally take into account besides the radiator thickness.
A high FPI count can require high static pressure fans and not even be 45mm thick, so really there is no Rule of Thumb without taking FPI into consideration, so whoever told you that is 100% wrong.