update: First of all I want to say that by definition, this is ALL speculation for all of us. And a good exercise at developing our deductive reasoning at that. But I have to admit I am having a hard time making sense of what your saying just above. On the one hand he (your source) is recognizing that they died with food in their stomach and mouth. But then says because it is only Mammoths and Rhinoceroses it is indicative of what? I am not sure what your actually saying there. My take would be because of their huge size, they would be the most easily 'survived' specimens, ie. when whatever 'swept through' they might have been the only thing big enough to stay in one piece. And I say swept through, I don' see this as an isolated incident, these Mammoths are being turned up everywhere, it had to have been huge whatever it was.
Also I read somewhere that the type of plant was specifically from a hot temperate or warmer climate, which is why it was such a mystery. I don't really get why he thinks it is 'typical' finding, not one but several of these Mammoths with the vegetation still intact. I do agree, they may have drowned first, the food in mouth had to have been an 'immediate death' scenario. Then regardless how well they froze, it still would have been counted in hours, a huge Mammoth would still take several hours maybe a day or two to completely freeze, so some decay wouldn't be inconsistent with being 'transported' to a freezing environment. but the freezing couldn't have taken more then two or three days, otherwise they would have been stinking bloated carcasses, and the idea of even sampling a small portion would have been out of the question, and the vegetation would have also been greatly putrified.
By Flash frozen I was just making reference that one moment they were eating, and the next they are dead, and at the very least the 'freezing process' must have started right away, although it may have taken maybe more then a couple days, to completely freeze an elephant. But it must have started within the hour to prevent the veggies rotting in their mouth and stomach. It had to be hours (maybe many, but still hours) instead of days.
I heard about them sampling the meat, but never gave it that much thought, never heard about any 'banquet', what I read just gave the impression, it was done on a lark, to 'record' for posterity, to highlight that there it was still existing flesh, which is quite amazing in itself given how many thousands of years it must have been there. I don't think anyone would seriously consider 'feasting' on some unknown animal carcass.
I am not entirely clear what your source is proposing, as it sounds too vague to understand exactly the picture he is painting. It sounds like you are saying they got hit by a mudslide as one possibility, and then were frozen. I still think the pole shift would still fit with this scenario. I will conjecture that whatever happened it was far ranging, else they wouldn't be turning up so many intact carcasses to this day.