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...)
