Not sure I can explain it better but let’s try this.
Cool fuel is obviously denser but cool fuel is also less prone to other problems such as premature vaporization as a result of pressure changes both within the carb and at its base. This is where atomization comes into play and making the fuel droplets as small as possible is the goal. Essentially only the surface area of the droplets burns so the smaller the droplets, the greater the surface area and in turn, the more fuel that’s in a position to be burned. The smaller sized droplets also tend to travel more freely and not drop out as easily while traveling through the intake runners. This all equals the potential for more power with less harmful emissions.
The new cars with the plastic intakes are also going for a cooler air charge as the aluminum intake manifolds tends to heat up that same air more but remember these setups are not carburetted and the fuel is being injected either at the head or directly into the combustion chamber. Cool air and cool fuel at this point. As Frank mentions, cool air carries with it a higher concentration of oxygen and that in itself is more power when coupled with the correct ratio of fuel. But higher temperature thermostats has the cylinder heads hotter which then aids in the final combustion process.
Blocking the exhaust crossover ports on our carburetted engines does make for some cranky cold engine starts but as both the engine and the intake manifold warms up to operating temperatures, then that heat alone is enough to aid in the heating of the fuel mixture. Hope that helps.