Monopolies aren’t based on the mere existence of competition. It’s based on power and market share. Eg, Chrome has a monopoly. Firefox, Safari, and a few niche browsers exist. But Chrome is the utter vast majority of the market and has pretty much all the power on dictating web standards as a result.
Microsoft had competitors when they got sued for their IE + Windows monopoly. But they had an utterly massive amount of the market share and used that to push their own browser.