Match history lists Kennen as getting ultimate at level 4
League of Legends Match History
The official match history website shows skill levelling order. Kennen in this game is listed as levelling his ultimate up at level 4, rather than at level 6.
3 Comments
He didn't spend his other points. He saved his points until level 6 because he was doing well in lane to give him a surprise burst so that the enemy team wouldn't counter him as bad. 
You can save your level ups, meaning that he could've put his 4th point into his ult rather then using them up to make it his 6th.
What Auspicious said. He probably didn't level up any abilities from level 4-6 then level'd his ult and then the other abilities. Sometimes I wait for when I feel like I need to use an ability early to level up first(Q, W, or E) but its become a habit and sometimes I carry it on into the later levels and even wait up to 2 levels before realizing due to the game being passive for a time. Kennen could have done the same thing there.