There is no question that air can enter the fan/vent with the cover on. However, that air is _greatly_ reduced. This is further show in that the heatsink (where that air vents) does get hotter with the cover installed. The differences is 92F without and 103 with it on.The Mavic on a hot day, lets say 32C as I'm in Thailand right now filming there, can get around 45C when idling with the cover on. It doesn't actually get hotter when flying because it's a computer and not a driving motor for the Mavic. So with the movement, the ventilation improves, even with the cover, so the Mavic cools down a little. Most of the heat is dissipated through the bottom anyways. The bottom is a massive heatsink.
So again, there _is_ less air flow with the cover on and the heatsink does get hotter for this reason. Does 92F - 103F make a difference? I don't know. it may not.