Depends on how one defines "monopoly". Back in the late 1990s, one could theoretically install non-Windows OSes like Linux, Unix or even purchase an Apple Mac. But yes, in practice Microsoft commanded the market and abused its market power.

Also it's market-specific. As the other commenter noted, the market share is region-specific and in the EU higher than 13%. In the U.S. it's even higher at ~47% (https://www.statista.com/statistics/236550/percentage-of-us-...).

You could argue that users technically have a choice to switch to Android but due to network effect (e.g. your family/friends all use iOS) and lock-in features like iMessage and Facetime, it's often a false choice.