So were most aristocrats of the time. Applying presentism doesn't invalidate the idea.

But there were also abolitionists at the time, even amongst that class. Jefferson not being among them does, actually, diminish his standing and his views on justice. This quote, for example, does not acknowledge that there are also laws which are unjust to obey; such as the owning of human beings in chattel slavery.

I don’t think suggesting that his quote would imply his slaves would be justified in violating their own enslavement is any kind of presentism.

He never actually said it, either.

It is just hypocritical: even his time most people knew slavery was unjust.

There were many abolitionists who strongly disagreed with him at the time, who were on the right side of history and morality, and he was on the wrong side. Just like you're CURRENTLY on the wrong side of history and morality today, by arguing that Jefferson was right to hold slaves in the past, to defend breaking the law in the present. What the fuck is wrong with you?

I bet Thomas Jefferson never imagined a convicted felon as President.

… in a society where slavery was legal, widespread, and rarely questioned.

Murder has never been legal.

The legality of it is not in question (the purpose of this quote). It was as unjust then as it is now.