Stealing what? I own my legal copy of Ars Fatalis which came as a free with a PC gaming magazine from Spain. Now I just use the game data with Arx Libertatis. No one was ripped.
If Ars Libertatis was complete they should have to create their own complete underground story replacing every asset and lore by hand, kinda like FreeDoom/Blasphemer and so on.
An actual libre engine reusing propietary data in a illegal way would be uMario, as it has literal ripped off BMP images pixel per pixel depicting SMB for the NES. And yet the game engine being GPL or MIT would be legal but the bundled game data is not; the creator would just have to use (and state the clear CC licenses) the copyleft artwork from Secret Maryo Chronicles or whatever it's being called today and everything would work as is.
Instead of Mario you would reuse SMC sprites being adapted for the contraints (pixel perfect for feet for instance)
and so and the only issue would be that the levels themselves would be copyrighted.
A single level as Supertux2 does is not a copyright issue because well, it's just a single one and a clear homage to the first level of SMB and even the level is named like that. It might fall under fair use, if I were Nintendo I woudn't sue them because unlike uMario, Supertux2 did things in a respectful manner.
Bear in mind that OpenTTD never did what uMario it's doing. When it had no open content you had to point to the copyrighted data yourself, be from the demo, be from the full game. Later OpenGFX and OpenSFX were created to replace every commercial asset and now OpenTTD has a downloader to get all the CC assets yourself without needing no commercial data at all.
The open content doesn't even have the original levels from TTD as uMario does.
If Ars Libertatis was complete they should have to create their own complete underground story replacing every asset and lore by hand, kinda like FreeDoom/Blasphemer and so on.
An actual libre engine reusing propietary data in a illegal way would be uMario, as it has literal ripped off BMP images pixel per pixel depicting SMB for the NES. And yet the game engine being GPL or MIT would be legal but the bundled game data is not; the creator would just have to use (and state the clear CC licenses) the copyleft artwork from Secret Maryo Chronicles or whatever it's being called today and everything would work as is.
Instead of Mario you would reuse SMC sprites being adapted for the contraints (pixel perfect for feet for instance) and so and the only issue would be that the levels themselves would be copyrighted.
A single level as Supertux2 does is not a copyright issue because well, it's just a single one and a clear homage to the first level of SMB and even the level is named like that. It might fall under fair use, if I were Nintendo I woudn't sue them because unlike uMario, Supertux2 did things in a respectful manner.
Bear in mind that OpenTTD never did what uMario it's doing. When it had no open content you had to point to the copyrighted data yourself, be from the demo, be from the full game. Later OpenGFX and OpenSFX were created to replace every commercial asset and now OpenTTD has a downloader to get all the CC assets yourself without needing no commercial data at all.
The open content doesn't even have the original levels from TTD as uMario does.