Hey-y-y-Abbey!Slowing coolant flow thru the radiator allows the transfer of heat in the coolant to the radiator tubes therefore the cooling fins cooled by the passing air, thus lowers coolant temp. That is a fact Jack! The restriction of the t-stat doesn't increase pressure the radiator cap does. That is a fact Jack!
You got those facts right! Also, the only time increased flow rate would increase heat transfer is if the radiator was so efficient that delta-T was extremely high. Then the radiator could handle the extra water and pull down the temp.