The same effect can not be visualized when you ferment under pressure (spunding) which also pushes from the top down as gravity so when you have a real clear wort with as little trub as possible it will in turn push the smaller particles downwards to settle in the yeast cake.
I see a couple of things that don't quite add up...
First, expanding CO2 doesn't necessarily exert pressure directionally, though it may be visualized as the CO2 pocket gathered at the top of the top of the vessel forcing the beer into a smaller volume, but as the CO2 becomes suspended in solution (it starts out more or less suspended during fermentation, being produced a molecule at a time), the headspace should equalize at the same pressure as the liquid.
Second, if the pressure were indeed acting on larger particles in concert with gravity to push them down, logic would tell you that it would be even more effective at clearing trub and you wouldn't have to worry about removing it at transfer from the keg.
Third, we all pressurize beer eventually when we put it in a keg or bottle and it continues to clear by the same mechanism as you're describing during spunding, so there doesn't seem to be a special case relating to clearing during fermentation under pressure. The only difference I can see there is that if all the pressure from CO2 from fermentation were contained (it isn't) then the action of rising CO2 escaping into headspace and out the airlock wouldn't allow particles to be borne up in the lava-lamp effect we see during active fermentation and they might stay settled. As it is, some of the pressure is released from the spunding valve which is a regulator, not a stopper and the gas still rises to the top, creating some of the the agitation.
Once CO2 pressure equalizes and can't escape, or once the beer is in a keg and there's no place for CO2 to go except into suspension, it ceases to attach molecules to particles and push them upward, so they settle. I think being cold (CO2 being more easily suspended in solution) helps more for clearing than extra pressure.
PS...I'm not trying to say you're wrong, just interested in exploring the technicalities.
I think there are some advantages to spunding, but I'm not sure that the wort-clearing thing is making sense for me.
