Right. It is a weakness and makes the meta analyses super important. I can't see all but trained athletes doing even low zone cardio for that much time. But I can see an active person walking, moving around, 12K steps, with a couple hours of genuine workouts / wk hitting that threshold. That matches with how I understand the rest of the studies on this better as well.

The easiest thing for people to do if they aren't confident about their level of stress (moderate vs vigorous vs maximal) is to wear a smart watch with a HR monitor. They aren't perfect (chest straps better yada yada) but you can see your HR zones and if you are in Z1 you are moderate, Z2/3 vigorous.

The language isn't that precise because a trained marathoner is doing 7 minute miles for two hours at 50% of the populations resting heart rate.

I skimmed the study rather than the article about it and I don't see them define it at all. They just had a machine learning model take accelerometer data and classified it into "sleep, sedentary behaviour, light physical activity and MVPA". Whether any form of walking counts as light or moderate in this classification is really anyone's guess