[BerislavLopac]

Not really. That more applies to board-game rules; TTRPG systems are more like guidelines for GMs and players.

But regardless of that, open source licences have nothing to do with what the code is doing; they are about using certain specific code in other products. Code is not copyrightable, so you can take some closed-source code and replicate its functionality in a different way - using different language idioms, or a completely different language - completely legally.

[kaoD]

Code is copyrightable, https://en.m.wikipedia.org/wiki/Clean_room_design

It might not be patentable, except it is under some jurisdictions https://en.m.wikipedia.org/wiki/Software_patent

[BerislavLopac]

You're right, I meant to write that the code behaviour is not copyrightable.