The problem with these types of "evidence" (and my quotes aren't suggesting this isn't real) has been touched on in these threads. CO2 doesn't stratify perfectly, resulting in discrete layers of pure gases. (Those pesky Dalton and Henry fellows again.) In purging a keg we want the gas in the keg, or at the interface with the beer, to be 100% CO2 and 0% O2. In the situations variously cited involving burping lakes or other natural phenomena, the CO2 concentration only has to rise above 3% to be lethal. That may be in the neighborhood of the best you can get with stratification-based keg-"purging" strategies for all I know, though I don't have actual figures; suffice it to say, it is a concentration below 100%. So yes, stratification is real, but a red herring with respect to the topic of keg purging.