He doesn’t say that. He said “improving working conditions for slaves is not an argument for slavery”.
He then goes on to say that 19th century slave-owners argued that slavery is good, because “slaves were better treated than wage laborers”. Which is an equally ridiculous argument in favor of slavery.
The idea that slaves had it better than workers is and always has been blatantly false, as evidenced by the many black people fleeing slavery in the south to be workers in the north and the non-existent line of northern workers attempting to become enslaved in the south.