Yes. Very true.
In fact, I would take this further. Slavery disappeared all across Christian Europe during the "dark" and middle ages. There were serfs and peasants tied to the land, but not slaves.
What ended slavery was, in short, Christianity. When the Conquistadors went to South America and enslaved the Indians they found there, the Pope held a meeting and decreed that these slaves would have to be freed. So, the Catholic Church was opposed to slavery.
In other words, what Steyn says is true, but what he leaves out is that slavery vanished all over Europe and then was REINTRODUCED during the Renaissance, when explorers went out and encountered the slave trade in Africa. Slavery was universal in all places and times except where real Christianity was dominant, whether Catholic or Evangelical.
There was slavery during the middle ages, but it was imposed on captured Christians by Muslims. Christian nations did not have slavery, as such, until the early modern period.
Wilberforce represented a pure strain of enthusiastic Christianity, and you might say that awoke the souls of Christians who had gone to sleep during the Renaissance and the so-called Age of Reason.
I should qualify that and say that Christian nations did not USUALLY have slavery. But it disappeared gradually. In fact, the Bible does not explicitly outlaw slavery. There were Christian slaves and slaveowners in the early years of the Church. But the implications of all men being created equal, and the implications of a Savior who died for all, whether Jew or Greek, slave or free, inevitably led in that direction.