To be clear if anyone has any doubt, antitrust law talks about firms having "market power". That's all that is required; the ability to move the market in bad ways. A monopoly (strict definition) is not in any way required to violate antitrust law. You could violate antitrust law with a 5% market share.

People wanting to nitpick about the word monopoly should be educated (first offense) or mocked and shunned (repeat intentional offenders).

Google has a 73% share of online advertising, according to a random study from 2019.