Amazon's "main business" is AWS, not their online store. For many years their store operated at a loss. Don't get me wrong, amazon.com is a HUGE business, but it's not Bezos' breadwinner.
AWS is a very large business, and much more profitable than selling physical goods online.
But Amazon's overall revenue was $56.6B (Q3 2018[1]) and "only" $6.68B of that was AWS. By comparison, revenue from Amazon's advertising business is $2.5B - and no one is claiming that is their main line of business.
Even the article you point to shows Amazon made a profit of $1.69B on North American ecommerce vs $1.35B profit from AWS.
So yeah - AWS is a high margin business, but no where near their "main business".
their main business is their online store, your linked article proves as much. If the online store was to disappear tomorrow the value of Amazon as a company would decrease much more than if it were AWS. It is proportionately more profitable but that doesn't make it the main business