Maybe. But whatever was being paid toward the mortgage can be divested for even greater gains going forward. Plus they're probably saving an addition $100k in interest.

If you have a a low interest rate then it can make sense to put more money into investments that will earn more than you're paying in interest. But in this case the payoff amount was pretty low and they had the cash so they can make way more over the next decade than they would if they continued paying the mortgage and trying to invest after that.