From the website you first posted, towards the bottom with photo confirming.
The Wiang Ping Super VIP Class Bus
Every day there is one trip of a super luxurious bus departing from Bangkok and Chiang Mai. It’s called the “Wiang Ping” Super VIP Bus. The bus is a 20 seater luxury coach aims to give its passengers the comfort feel comparable to the first-class airline experience.
The seats are bigger and more spacious. There are only two rows of seats, separated from each other by the aisle so that you won’t be bothered by a chatty seatmate