Monopolist does not mean actual monopoly though. Also from the FTC:

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.

https://www.ftc.gov/advice-guidance/competition-guidance/gui...