I absolutely guarantee your students do not hold that position in the way you think they do. Sure, they acknowledge past people had different ethical standards, but only that those people hadn’t yet progressed to what they perceive as the self evidently *correct* standard.
Wanye's annoying conceit is that he's the liberal whisperer. He can read the minds of liberals and describe their strange ideas to his followers!
But his liberals are straw men created to feed the vanity of his audience that they are smart, and liberals are dumb.
It's a popular shtick, but it's dumb.
(And wanye should know better. He's not a dumb guy.)
You could find some liberals who fit wanye's caricature of all liberals, but most liberals (like the students I teach) agree that people in the past had a different understanding of morality. This is just nutpicking.
There are fair generalizations you can make (and criticize) about liberals (and progressives, and conservatives), but that's not the game wanye's playing.