That HN poster is dead wrong.
Amazon’s retail number counts only the stuff Amazon sources and sells itself. If you have done any shopping on Amazon in recent years, you will notice most of the stuff on Amazon.com is sold by 3rd party sellers. For those sales, Amazon only counts the fees it charges the sellers, which is much lower than the gross sale receipt for 1st party sales, but at much fatter margin.
Amazon never broke out how much business 3rd party sellers are doing as it only reports the fees it charges. I have seen estimates pinning that at least as big as 1st party sales and very likely higher. So Amazon retail size is actually more than 2x reported.
Amazon’s FBA division is actually much more dominating than AWS as it has zero competitor at scale. Amazon is an online retail platform, Walmart is just a retailer.