Watch it again they do not use a mixed grip because they are USING Straps. If you are using straps there is absolutely no reason to use a mixed grip. Using a mixed grip and straps makes it very hard to lock the straps in nice and tight. Now go and Youtube a Strongman Deadlift where straps straps are not allowed and I guarantee you 95% are using a mixed grip and if they are not its a hook grip but, you hardly ever see it ( I have personally NEVER seen it ).
Look at every Powerlifter. They are the ultimate ones to look at when Deadlifting and you have not seen many great Deadlifters use a hook grip. Dan Green, Brandon Lilly, Andy Bolton and Benny Mag all use a mixed grip UNLESS they are using straps in training in which case they all pull double over. Your not thinking the WSM theory all the way through.