Sounds like bunk to me. Engine management systems inject more fuel on a cold day because cold air is denser, and therefore contains more oxygen molecules per unit volume (and the airflow sensor only measures volume).

Without the extra oxygen, more fuel is likely to do no more than make the mixture too rich, with the extra gas getting taken out by the catalytic converter rather than being burned in the engine.

Still, there are a million other variables (atomization, mixture distribution, etc.), so you never know....