The " the Merredith effect" was first put out in a paper read in 1935. It was the same paper that advanced the idea of jet thrust from the engine exhaust. The theory was hardly a secret.
The problem was in turning the theory into reality.
You needed a radiator core that was of the right size (cross section) and thickness to have the low pressure drop through the radiator and not so large or thin to limit the amount of heat transfered to any given unit of air.
You also needed air ducts leading to and from the radiator of the right size and the right change of cross section to keep the pressure changes and air flows with in the limits they needed to be order to get any usable thrust from the whole system. Or even a significant reduction in drag.
too short a duct or too abrupt a change in cross section of the duct could ruin the anticipated results.
With a real shortage of wind tunnels and such to experiment with before the war and little experience to go on it was going to take a while (or inspired guesses) to get it right.