What is the alleged anti-competitive behavior? The article doesn't mention it. It sounds like she is calling for their breakup merely for being big and powerful, and if that were cause enough to break up an entity we would logically start with the government.
For starters, Amazon provided a marketplace platform for third parties to sell goods. They then gathered data on which items sold the best, created their own line of products based off this data, and crushed their customers-turned-competitors using their massive advantage in economies of scale.
Batteries, clothes (lots of clothes), backpacks, these are all products that are cheap to produce and where brand loyalty plays a large role. People generally trust the name Amazon. It's not rocket science to realize people want cheap clothes and batteries.