To my knowledge, they get around the legality issue by not actually distributing the textures, but rather just a script which happens to work on the Minecraft textures and which does the conversion locally on your PC.
From a quick bit of a research, I found this, which seems promising: https://forum.luanti.org/viewtopic.php?p=443213#p443213
But yeah, in theory any Minecraft update could break that, so you'll have to see if it still works...
all 2 comments