I may be writing about pressure gradient but if so, only by accident. It's quite clear the "Greg" piece is about a density gradient. The whole argument rests on the idea that there is a density gradient present and the consequences of that is that there are more molecules at the bottom of the balloon than at the top and the consequence of that is even if each molecule hits the balloon with exactly the same force irrespective of its position, the the overall effect is more forces applied at the bottom than the top.
My intuition today is that this can't be right. And many people over the years have queued up to say it isn't right. But I'm not sure why it isn't right, it sounds right, but none of them so far has gone beyond "that is not right". Nobody seems willing to able to explain exactly why it isn't right in terms of the behaviour of individual molecules. There must be an obvious error or an unspoken error or something is omitted but if that is the case, why can't people just explain it very simply: "Those 4 molecules at the bottom that provide the lift in this model don't exist, or their forces operate at right angles to the direction the model suggests", or something of this nature. My take away from this situation is that there are a lot of people who struggle to accurately describe physical situations, even those who believe they have a good understanding. Probably because physics is subtle.
No doubt, when someone finally hits on the correct microscopic description, it will be face palm time and I'll be saying "Ah, of course, that's how it works. How obvious and how silly of me to think otherwise". I'm looking forward to that moment of enlightenment but I've been waiting 20 years so far, patience is a virtue...