Aha!
An excerpt from the article comments:
“The Pfizenmayer book is available online at:
Les mammouths de Siberie; Le decouverte de cadavres de mammouths prehistoriques sur les bords de la Berezovka et de la Sanga-Iourakh - Digital Repository of Scientific Institutes
It's in DJVu format, and I found the HTML5 browser support painfully slow, so you may want to try one of the other options. The section on the mammoth's food starts at page 139 [= page 146 in the online viewer].
The relevant passages seem to be (my translation):
"The remains of forage in the mouth, just like the partly-intact food remains in the stomach (of which remains we were subsequently able to conserve about 15 kilos) consisted exclusively, as I have already said, of herbaceous species and larger flowering plants which still revealed parts of seed capsules; on the other hand, there were found no conifer remains, as a result of which it must be admitted that needle-leaved trees did not feature in the menu of our pachyderm.
Later, at the botanical museum of the Academy of Sciences, there was success in identifying a number of the plants in the stomach contents.
...
One could identify, among the various representatives of this northern flora,
some sedges (types of carex). Among the higher flowering plants, one could clearly recognise thyme (Thymus serpillum), a labiate very widespread on the heathlands of Europe; then the yellow poppy of the Alps (Papaver alpinum var. xanthopetala);
the bitter sharp buttercup ["renoncule amère tranchante"] (Ranunculus acer borealis) which is also preserved in the Alps as a survivor of the glacial period; a species of gentian and
a species of cypripedium (orchid); Thalictrum alpinum; Atragene alpina var. sibirica and various other plants which are still to be found today in northern Siberia.
15 kilos of pollen is most improbable, so I think substantial amounts of the actual plants (though not necessarily the petals) were preserved.”
And the author of this article that you posted, Jason Colavita, acknowledged in reply:
“Isn't it interesting how nobody ever seems to know what they're talking about once you get beyond the first generation or so of copyists? It certainly sounds, though, like the remains weren't in a state of preservation to qualify as freshness-sealed, though apparently in much better condition than some later scientific writers assumed.
The buttercups aren't in the mouth, though, and
seem to have been frozen alongside all the other stomach contents. I wonder why, other than the fact that they are flowers, fringe writers seized on the buttercups and nothing else, not even the poppies?
I wonder if the discrepancy in the later scientific accounts stems from
the plant remains decaying after they were thawed out, leaving only seeds and pollen to study?”
That’s it.
Where’s the debunking attribution?