I see no source code, nor an open data set. I do not call that an open database in any sense of the word.
But I realized that by using a session token, one can actually enumerate the database easily: https://gist.github.com/lgommans/1b766fd7a24b85f56be13b37c4400218 (At first I thought that one would have to keep requesting random questions until your unique count matches the one mentioned on the website, but using a token fixes that.)
Still missing is a version number for the whole set and an API call to get updates. If the developers want to, I'll implement it for them in their code (it's PHP, right?), provided that my modifications are under a FOSS license (the rest of their code can stay proprietary).