Look, here is how to view the heatsink and CPU interface.
In a perfect world, the heatsink and CPU would have perfectly flat surfaces with no microscopic pits and valleys in their surface. The clamping mechanism would apply exactly equal pressure on the entire mating surface. This is the best-case scenario and the most efficient heat transfer is achieved with simply having the bare metal surfaces together. This is the key principle that we are dealing with here. Metal to metal provides the absolute best heat transfer. Period.
However, in our imperfect world with our imperfect surfaces and clamping mechanisms, we need to get as close to this ideal scenario as possible. The solution: a thin layer of highly efficient thermal transfer material. The perfect layer connects the parts of the heatsink that aren't in contact with the CPU to make contact but does not insulate the parts of the heatsink that do touch the CPU. Any extra thermal paste that prevents the metal-metal interface is actually reducing the efficiency of the heat transfer.