I responded to your comment before, but didn't sufficiently think it through, so I deleted my previous response.

You raise a good point, and they do indeed acknowledge this flaw in the study:

One limitation of this work is that it was based on 1-day diet recalls, so our results do not represent usual intake. Averaging both days of data available on the NHANES would not address this problem, would reduce our sample size by 15%, and would mix recall methods between an in-person interview (day 1) and one done on the phone (day 2). Still, as a check, we examined day 2 and found the same associations with gender and MyPlate guidance.