I feel these days that it's more correct to say Amazon was first and foremost a retailer, but these days they are just UberMegaCorp across so many businesses (Amazon site proper, AWS, WholeFoods, Kindle, Echo/Alexa, movie studios, etc. etc.) that it's hard to say they are foremost a retailer.
I have a similar question, though, in that I've heard lots of nightmare anecdotes about Amazon code quality, but obviously whatever they are doing is working on some level.
There a plenty of things that decouple incompetency in their software org from business success:
1) Amazon has been incredibly successful frontrunning two MASSIVE verticals: online retail, and cloud computing. They were early winners and invested an INSANE amount of money to be #1. That gives them a lot of leeway to fuck up and still be on top. Even if they stopped doing anything at all but maintaining whatever shit show is going on behind the scenes, it'll be a while before they aren't #1 in at least one of those two huge markets.
2) Tech debt doesn't hurt you today, it hurts you tomorrow. You can cut corners and get products out the door fast, but eventually features and products that should take X amount of time, instead take 5X. Given their lead in their units heavily reliant on software, I doubt it would hurt them anytime soon.
3) You can shit on a lot of people for a long time before it hurts hiring when your stock has shot up the way Amazon's has. Money will make people do dumb things.
4) It takes a long time for poor hiring and management practices to incapacitate an organization the size of Amazon. Turn over at the bottom may be high, but I doubt it's the same at the top, at least up until the past few years. When leadership starts turning over more you'll see a lot more business mistakes.
So on what basis ia Amazon incompetent at software? It sounds like Amazon is results driven, and pretty well-unit-tested code isn't critical for that.