Source: Two family members in their mid-50s who have had double knee replacements due to them being overweight their whole lives.
Looking at a 4 kg bag it would be so hard to eat anywhere near one bag every day, I just could not imagine getting through that volume.
IIRC from last time this was posted, the hypothesis for why most people felt really good on this diet was a nutrient deficiency in most diets that's addressed by potatoes
The author doesn't claim potatoes are low calorie, he says calorie for calorie, potatoes make you feel more full.
I can see it, eating one large potato will make me feel full longer than eating twice as many apples.