Amazon doesn't control where you buy random stuff, but they have disproportionate influence in some areas, like books. I'm pretty sure they are already in the position to make or break some brands simply by banning or promoting them.
Consumers prefer Google and Amazon, and it would cost them little to switch. Most consumers have no choice but to accept AT&T or Comcast.
When Microsoft was ruled to have violated antitrust laws in the early 2000s, there were alternatives available for browsers (eg Netscape) and operating systems (eg Linux and Mac OS). However, in the US court's opinion, Microsoft made it too difficult to install competing browsers on Windows. I believe that the courts also frowned at Microsoft forcing OEMs to refrain from offering competing OS products. Thus it was found guilty of antitrust behavior, and was forced to settle with the US government.
(Thus IMHO the original article seems to oversimplify things. In the Microsoft case which is well after the "Chicago school" influence was around, low consumer prices were not really the driving factor in the lawsuits. Also, the "Chicago school" may not be a single monolith opinion. One of the first articles I found on a Google search for current Chicago school antitrust feeling was a Bloomberg article about this paper -- https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3129221 -- which is about the negative effect of corporate monopsony on the labor market... so...)
But you do have to have done something that is anti-competitive in some market. (Not only that, but you have to have a competitive advantage in the market in which you take the anti-competitive action.)
Merely being better than the competition is not the traditional (outdated) definition of monopoly.