As @sreconomist notes, in the same speech Biden was discussing the worldview of the Unite the Right protesters at Charlottesville and there's a chance this line was part of that:
![](https://pbs.twimg.com/media/ENPk37aWsAMjRf2.jpg)
It comes in a segment about fighting abuse on college campuses and it seems Biden is actually talking about how "our English jurisprudential culture" has made this an intractable problem in America.
There's a belief -- apocryphal, I think? -- that "rule of thumb" comes from an English common law provision that let man beat his wife with a stick thinner than a thumb.