He also did it on a kids bike the year before: https://www.youtube.com/watch?v=49V4P_wkKTU
I should mention he currently rides for a UCI ProTeam, which is one tier away from the highest level in the world.
So, some more context: swapfiets is the dutch student bike provider, they are very basic bikes, no gears, no nothing because what isn't on them can't break.
Key quote: "What do you miss the most about a racing bike?" -> "A slightly lighter gear".
He's doing remarkably well by the way, testimony to his skills, keeping up with those guys on their carbon frame racers with 22 gears is extremely impressive even over a short distance. The gearing on a typical swapfiets is typically 46/16 or 44/16 so ~3:1, on a racing bike you would have access to 36/23 or even 34/23 on a special climbing gear set. So he has to work about twice as hard to maintain the same speed.
Passing bike comments 'you're crazy man' :)
Nevertheless, the lowest gear on that bike is still not much help on a steep ascend like this.
One thing that's not mentioned in the article is to use Couch Surfing and Warm Showers. Warm Showers especially is amazing, because it is an amazing cycling community and you can borrow anything from small stitching kits to a full-on welding kits to fix bicycles. Full of amazing things and people in almost all euro velo routes.
FlixBus will take bicycle on board on most buses (for 10 euros IIRC), and most trains will too on weekends and off-peak hours.
Overall, it was a fantastic experience and I can't recommend it enough.