> This seems like it would be an issue for any studies relying on absolute food consumption being accurate.

Exactly. Those studies either don't get done, or when they're done, they produce garbage results that get ignored or get interpreted as diminishing the importance of absolute food consumption.

> it doesn’t seem particularly relevant if you believe everyone is underreporting their food intake

It says that virtually everyone underreports. It doesn't say that everyone underreports equally, and there are good reasons to expect this not to be the case. If embarrassment is a contributing factor, for example, you would expect people who are more embarrassed about how they eat to underreport more. If people remember meals better than they remember snacks, people who snack more will underreport more than people who snack less. If additional helpings are easier to forget than initial helpings, people will underreport moreish foods more than they underreport foods that are harder to binge on. With so many likely systematic distortions, it would be surprising if everyone underreported equally.