"One thing became very clear, and every mountain biker will certainly be able to confirm this: when riding uphill on a racing bike, there is always the conflict between riding standing up, which puts more pressure on the pedals, but you have to keep lifting your weight onto the pedal, and riding sitting down"

This is simply not what riding uphill on a roadbike is, unless you are not used to that position at all. The switch between seated and standing is not done to push harder, it's done to get a bit of variation in position and muscle use. It's not a conflict, it's an opportunity.

Except for those particularly steep slopes where you need to move your center of mass forward to not literally tip over, where it's not an opportunity but the only one. That threshold is sits in the vicinity of 20% incline, depending on size and seating position. This is with a wheelbase about twice as long as the kwiggle, and with a considerably lower center of mass. The author even seems to emit that, in a way, with the suggested tweak "So if you ride the Kwiggle in the mountains, always place the saddle a bit more forward". Good luck descending afterwards if it happens to be a route where you might actually need to brake.

The difference in the climb the author was so enthusiastically describing is simply that he went up more than 10% slower on the kwiggle than on the roadbike (92 minutes instead of 81). If you have ever measured endurance efforts in that game, for example on an ergometer, you will know what a huge difference 10% means.

I mean, nice publicity stunt and it surely was a lot of fun given the weather, but if you want to do that more, get proper equipment.