There's a difference between a new entrant appearing, and a firm with a monopolistic presence in one (or more) areas extending vertically into others.

Note that Amazon has a distinct and proven track record of utilising its monopoly position to squeeze competing and complementary firms in other spaces. Notably with Hachette.

http://www.latimes.com/books/jacketcopy/la-et-jc-amazon-and-...

I believe in the implication is that Amazon will cut out the competition because of its cost advantage, leading towards a monopoly, not unlike what it's (trying) to do in many other sectors.

"monopoly" is just a moral classification for something that is "big" now. It doesn't mean lack of competition or have any other strict definition anymore.