The question isn’t so much whether a company is a monopoly, or part of a duopoly, or oligopoly, but whether their market power lets them coerce their rivals, suppliers, customers, etc. It’s a common misconception that a company needs 100% of a market before they can exert monopoly power (as a seller), and the threshold is even lower for monopsony power (as a buyer), which is common in labor markets with powerful employers, for example.

Legal thresholds for application of anti-monopoly laws have historically been quite low as well. For example, in Brown Shoe Co. v. United States in 1962, the US Supreme Court approved blocking a merger between Brown, a company that manufactured less than 6% of shoes in the US, and Kinney, a company that sold only 2% of shoes! And that actually seems like the right approach, since the Clayton Act, for example, doesn’t only prohibit acquiring 100% of a market (which would render it worthless), but blocks any acquisition when “the effect of such acquisition may be substantially to lessen competition.”