Nick, yet again you have eluded the question - we all know that ground effect exists, the link you provided is to a report that shows reduction in rpm and increase in Cl for an aircraft in ground effect.
It does not say how it happens, only that it does.
If it is not pressure related then what is it - all the reitieration that it is not pressure related is not verified by any other evidence to the contrary. You can tell someone the world is round (alright an oblate spheroid) as much as you like but without proof they are unlikely to believe you.
If you do not know then say so instead of treating us like errant schoolchildren who don't understand their teacher (because he hasn't explained something properly).