They do their math in the article. "In the United States, the average diaper sells for about 25 cents. The quarters add up quickly. Newborns need as many as 12 changes a day. That’s $21 per week, or $84 per month. Bigger kids need fewer, but their diapers are more expensive." It's a worst case calculation biased towards newborns, which they don't remain all year, obviously. For me, it averaged more like $800 - $400 a year. Also, 2.99 a pack is insane for me as an American. That would go for about $8-$10 USD, depending on size.

Well in smaller EU countries the prices tend to be little bit higher due to quantities but year that 1000$ doesn't add up. ATM couldn't find anything under 0.18€/piece in Estonia. https://www.prismamarket.ee/products/search/teipm%C3%A4he

Same for Netherlands. The month-box of 210 huggies costs 34 euro, so you get a number around 300 euro/year. If you buy from stores with discount, you'll get a price of around 11 cents per nappy: https://www.luierinfo.nl/aanbiedingen/pampers/premium-protec...