That's both incredibly condescending and wrong. "One mega company owning every industry in the world" is not the only way to have a monopoly.
You can have monopolies on specific industries or in specific regions (e.g. AT&T and Comcast split up towns in a state and each one is the only provider in that town).
So was the parent's statement of "This is really basic" when they hadn't provided any evidence or even a clear claim (did they mean a few big players controlling each market, as in the fuel market, apple market, etc., or a few big players controlling a market as in the American market, European market etc).
That's both incredibly condescending and wrong. "One mega company owning every industry in the world" is not the only way to have a monopoly.
You can have monopolies on specific industries or in specific regions (e.g. AT&T and Comcast split up towns in a state and each one is the only provider in that town).