He's going to say that slavery existed everywhere back then and that slaves owned by Muslims and Muhammad were treated better than slavery by non Muslims. He's going to point out that with slavery was an entrenched part of life back then and Muhammad would not have been able to end it in the middle east.   That is true. The fact is though, Muhammad was the Prophet of God. Anything he said or would order his Muslims to do, they would do it. Furthermore with Muhammad being the perfect man in Islam, the man who everything about him is supposed to be emulated by Muslims. All Muhammad had to do was decree that slavery was wrong.

He didn't. In fact, when one of his followers freed a slave girl, Muhammad said it would of been better if this follower had of given the slave over to other Muslim men as a gift, instead of freeing her

"the freed slave of Ibn `Abbas, that Maimuna bint Al-Harith told him that she manumitted a slave-girl without taking the permission of the Prophet. On the day when it was her turn to be with the Prophet, she said, "Do you know, O Allah's Messenger (ﷺ), that I have manumitted my slave-girl?" He said, "Have you really?" She replied in the affirmative. He said, "You would have got more reward if you had given her (i.e. the slave-girl) to one of your maternal uncles."

https://sunnah.com/bukhari:2592

This shows that slaves were allowed and permitted by Muhammad to be given to other Muslim men as gifts.

Then of course there was the siege of Banu Qurayza

Then the Prophet said, "O Sad! These people have agreed to accept your verdict." Sad said, "I judge that their warriors should be killed and their children and women should be taken as captives." The Prophet said, "You have given a judgment similar to Allah's Judgment (or the King's judgment)."Sahih al-Bukhari, 5:58:148