Seats in this pirogue are not just for sitting they also support the hull sides. You can choose to install 1 or 2 seats. If 1 seat, it should be installed in the center of the pirogue. If 2 seats, one seat should be just aft of center and the other spaced far enough to accommodate another person.
The instructions provided on this website are not always detailed enough and are "my way" of doing things. Ask 10 different boat builders or woodworkers how to do 1 thing and you'll get 10 different answers.