One Important thing is "judge people by the standards of their time" always mean "judge people by the standards of the ruling class of their time" even if white Virginians had no problem with Washington's slaveholding, the enslaved people did.

It's a way of erasing other voices https://twitter.com/AnandWrites/status/1275448581214728193

Obviously, white opinion in the Washingtimes was already turning against slavery. It was a fringe opinion but there were already abolitionist petitions before congress by the 1790s.

If we're judging confederates by the standards of their times, we must acknowledge that a popular version of Battle Hymn of the Republic was "Hang Jeff Davis from a sour apple tree."

The standards of anytime are diverse, and you can almost always find a contemporary opinion which is as harsh or harsher than our retrospective judgement.

Giridharadas is talking about slavery in Ancient Greek philosophy and I don't know anything about that.