The argument isn’t that AI brings the labor cost down to 0 in isolation. It brings the labor cost of the same amount of production down. So you get more production (more things = more supply -> lower prices) out of less labor.

Its also the 'labor theory of value'. That's the economic theory that Communism is based on. It has never been accurate and wasn't even considered legitimate during Marx's lifetime. It has possibly the worst track record of predictions of any theory ever conceived by people. Yet somehow academics still reference it. Nobody who actually is impacted by making the wrong economic predictions does though. Funny that...