There is no way in HELL that this blanket statement is "always" true.
It all depends on the mechanical fan, it's size, blades, pitch, whether or not it has a proper shroud, whether the pulley ratios are helping, and probably a few other things
Electric fans are exactly the same way. Some aftermarket fans don't flow enough air even if installed "optimum", some have poor blade designs (so called reversible blades), some don't have provisions for shrouding the radiator and so don't pull air through the whole core.
