Looks like he has them bolted on to me. You can see the ends of the bolts across the top behind every tube. I don't know if those are store bought tubes or not, might be because they look flared but the ones in gander mountain and bass pro have holes drilled though the tubes one hole being bigger to allow the head of bolt to pass through.
Wonder if the bottom pvc is filled with sand or something because I don't see it fastened to the truck. Seems like it would blow over in the wind while driving or are those only used once you get someplace as a rod holder?