I don't want to mess with the heavyweights here, but if we're talking ground effect with respect to helicopters below translational lift speed the air is not stationary, there is an obvious downwards flow of air, that undoubtedly builds an area of higer pressure beneath the disc in proximity to the ground, because it's flow is restricted, and it's direction changes, meaning a force needs to act upon it (newton), which must be due to the fact the pressure in that area is greater than the surrounding ambient pressure (pressure gradient).
I don't get this mention of the requirement to be sealed on all sides etc... because just looking at wx pressure systems, high's low's etc... they are not sealed and there are obvious changes in surface pressure due to expansion/compression upwards/downwards movement of air (like above/under our disc). If everything needed to be sealed wouldn't the surface pressure be equal around the globe (perhaps with some fluctuations due to temperature)
I think that the principle of a reduced induced flow in close proximity to the ground below tx lift speed must hold some truth as a result of higher pressure under the disc... having said that I haven't noticed the altimeter drop on take-off before