I think you found a misinformed CM. We booked that tour last year for 7 people which was an odd number. Having done the tour, which is fabulous btw, I don't see any part of it that would require even numbers.
after 35min on the phone our family of 3 was booked for DTM tour -- the CM was confused!
Had to get another CM to help out and when I asked him about that policy he said she was mistaken and that it was a minimum of 2 that needed to be booked .
So we are all set for the tour and I am sure DS (age: 4) is going to have a blast
I think there's a certain price for the first 2 participants and then any participant after that is another price. Maybe that's why the CM was confused.