Wait, I have a question about this too. I went to download it and the download site read:
If you installed the game from Steam or GOG, the 1.21 patch is already included when you download the game and you don't need to manually apply it. I downloaded it from GOG.
So is Arx Liberatis already included?
No, that is referring to the fact that GOG's version is already patched to 1.21. All you need to do is double click the downloaded Libertatis exe, and it should self install, and you'll be good to go. Just to be safe, if you have space on your hard drive, I would save a copy of the installed game separately in another spot, until you are satisfied all went well and the game is up and running. I always try to do that anytime I install mod(s) myself. IIRC, this one installed no problem, but everyone has a differnt computer and system, so I always say better safe than sorry. (This mod is awesome btw)
edit: f'in typos