Well, do you think that the question meant, early Christians argued that the Greco-Roman gods were real beings, and sucked, or that the question meant, Jesus was real, and rocked?
The passage says they wanted to denounce pagans. Therefore, their argument did not involve Jesus, historical, or not.
Euhemerism, therefore, historicized mythological pagan gods, and early Christians, therefore, used this to attack the pagan gods. SINCE they are not historical, they can be MORE easily attacked than if they were mythological. Who is easier to denounce, Obama or Vishnu?
So, the attack would only work if... the pagan gods were humans. If they were anthropomorphic, that would be saying that attacking a walking turtle-god is easier than attacking a human. It's not. If they had no basis in fact, that is NOT using Euhemerism. If Jesus had historical reality, that does NOT attack the pagans, it SUPPORTS Jesus. We do not care about Jesus.
Personally, I do not care about Jesus, so this question was hilarious.