One can violate antitrust laws without being a monopoly. Certain parts of the law (regarding collusion on price setting, for example) can be broken by very small businesses.
This is true, but those are all cases in which putative competitors collude to essentially form a cartel. Which is a distinct category of antitrust offenses from anti-competitive behavior.
See this helpful FTC page: https://www.ftc.gov/tips-advice/competition-guidance/guide-a...