That's not true, but it's close; at least according to US law. Microsoft was deemed a monopoly in 2000 (which is actually legal) and abused it's monopolistic powers (which is not legal) while other OS's existed in the market: Apple, BeOS, Novell, Linux, OS/2, etc.
As you know, Microsoft had more than 90% of the OS market worldwide. While other OSes did exist, they didn’t provide meaningful competition. The person to which you replied meant that Google Milk is only a problem if Google abuses its search monopoly to push the market away from other milk vendors, not that Google isn’t a monopoly as long as another search engine exists. Their main point was that it isn’t abusing a monopoly to launch a broad array of products if each of those products compete in a fair market.
https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor....