It's common for people from lots of fields to go to law school, AFAIK. I don't see why a math degree specifically would be helpful in making that switch.
The claim was that a math degree is easier for making the switch than the opposite switch: e.g., an English major would likely do as well as a math major in getting a law degree, but would find it nearly impossible transitioning into mathematics without the equivalent of a second BS.
I don't think this is particularly controvertial.