Again, the whole premise of the article is that there really is such a thing as disproportionate beef eaters (DBE), and it spends time talking about this group

Again, the whole premise of the article is that there really is such a thing as disproportionate beef eaters (DBE), and it spends time talking about this group explicitly. So the wording doesn't suggest otherwise, it explicitly suggests this is a real group. Regarding the study this is a both can be true situation. There can be (1) a population who is disproportionate in their beef eating, and (2) a study about 12% doing the most on any given day can count in favor of that group being real and (3) not everyone from the daily 12% is part of the DBE group. It's more likely a venn diagram overlap, and where it doesn't overlap, people who aren't part of the DBE are incidentally in that 12% while being closer to average in the aggregate over the longer term. Those facts can all sit together comfortably without amounting to a contradiction.