The pack works in both versions, and i don't know why. Most of the files and items of the default resources have been renamed, so the pack should be broken in 1.13, but for some reason it works just as well in both 1.12 and 1.13, with one exception. Jasper made a new blockstate for vines which breaks in 1.13. Mabe that will help figure out what's going on? There were no new folders or items in the "minecraft" folder or the resourcepack folder itself, so I think it may have something to do with the title. Mabe the long combination of letters and numbers is a code that minecraft uses to make it work? I haven't tested it out yet, but I figured I would make this post first in case it doesn't work.
Specifically it's the code at the beginning. You have to put it at the beginning of your resource pack title for it to work.
But since I have seen no confirmation from Mojang that we are allowed to use this code, I would highly suggest you do not use this code in your resource pack titles and release them to the public. It may have been made specifically for Jasper so he could continue working on the pack through multiple versions. I will see if we can use it ourselves, but until I get confirmation, I won't be releasing anything using this code.