According to links I found on the page, it looks like the software being used is provided by Kiwix [1], and the file format used to archive Wikipedia (and other sites Kiwix archives) is ZIM [2].
I agree, it's an interesting project. I would like to see more specifically about how many users they can serve at once from a Pi Zero. Unless they just stuck Cloudflare in front of it?
[1] https://www.kiwix.org/en/
[2] https://openzim.org