His reasoning was totally wrong.
He started out talking about God's Commandments not being burdensome if we love God. If that is the case then marrying a Witness will lead to happiness. I did that but I don't have that.
Also, they focus on the negatives, but if it is not burdensome, there should not be negatives for marrying Witnesses, there should only be positives.
As for Deut if they wanted to marry NonIsraelites, why would Jehovah care? it is one or two people out of a million. And as for serving the Canaanite gods, that would mean they would choose the losing side. Did Hitler give any commands that his soldiers could not marry Polish people whom they invaded? Would the Polish want to get with invaders? Would the Germans switch to the losing side?