> According do US law, having 50% of the market can be enough.
Having much less than 50% of a descriptive market can be enough, if, e.g., you have pricing power, which demonstrates that irrespective of what other players may be described as being in the same market, they are not actually competing with you.
> Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors. That is how that term is used here: a "monopolist" is a firm with significant and durable market power.
Having much less than 50% of a descriptive market can be enough, if, e.g., you have pricing power, which demonstrates that irrespective of what other players may be described as being in the same market, they are not actually competing with you.