This might not be able to be 100% accurate. I think the general challenge database looks for unknown type caches with the english word CHALLENGE in the title. Some of these are just puzzles. I know it counts a few on my challenge list on my profile stats.
Edit: Jakuje and others mentioned that there was some cleanup done on this. There's still a few exceptions out there so still not 100% accurate. Appears to be a constant work in progress.
But I do like the idea. You'd have to take it with a grain of salt just like the TOP FTF list.