So? Just because people have done something for a long time doesn't mean it's right.

Or are you disputing the historical claim that humans on average have never been more tolerant towards strangers than today?

This seems like it is true but what's the evidence for this claim and why are you asking if I dispute it? Where did I say anything about changes in tolerance over time?

Tolerance for strangers is more in need of sociological explanation than the opposite

Why? What if humanity over time grows and learns thanks to our ability to store information, like in books.

More importantly why do we need an explanation and why does it matter?

If you are trying to use the fact that something happened for most of human history then changed recently and that makes it abnormal. Think slavery, rape, woman's right, etc