I've read this statement before on this forum, pretty sure it is incorrect. Increasing fluid velocity increases the convective heat transfer, which will improve the efficiency of the whole system, which results in lower average water temperature.
The statement is correct, which is why companies such as Moroso sell "water restrictors", that get installed in place of thermostats in race only engines.
The issue with no restrictor is that the coolant is allowed to flow so quickly, that it doesn't spend enough time in the radiator (and heater core, which is also a forgotten part of the cooling system), that the coolant doesn't spend enough time in the radiator to transfer the heat from the liquid to the metal (brass or aluminum) heat exchanger (radiator). It also works the other way, the coolant can flow so quickly through the engine itself, that the coolant has limited time to "pick up" heat from the engine itself.
Another issue is that the coolant can cavitate when allowed to free flow, which reduces the ability for the coolant to actually transfer heat.Air and air bubbles are a very poor conductor, and therefore have a poor ability to be able regulate temperature.