It uses Firebase.
It really looks very expensive for the functionality and 1.5K monthly visits.. But the developer seems to be fully transparent about costs.
When I built this site I really didn't think much of it and used Firebase just because it was really quick to build and I was getting only a few hundred visits a day. But since last Friday it has received over 35K visits hence the really expensive bill, which by the way is not so much for the storage, but for reading the Firestore database. I simply haven't had the time to re-build the site in a more efficient way it since then, hopefully tomorrow I'll get working on it and find a better alternative solution.
Based on information from the developer's website, it looks like they're still a university student, so probably using what is a cool and easy approach; they probably haven't learned a lot about load and scale. That Firebase bill could probably be written off as tuition :-)
I only used Firebase because when I built the site it was only a small scale solution, but now that is has blown I'm looking at alternatives because this is just a waste of everyone's money. And by the way, I only graduated from my master's last month, and I'm not even a computer scientist, I went to design school and have a degree in industrial design, hence my messy code practices. I'm working on them though, so if anyone is willing to give me a hand or some guidance, feel free to do so.
Most probably you're right, it's sad, costs could probably be a lot smaller and contributions could be a small but sweet and deserved compensation for coffee/beer :-)