i think they mean the exe was modified or something, not as simple as dropping in new content files like many mods are able to do in other games. you can't just open up a json and write some new stuff in to make your mod, you have to actually decompile and recompile the game, which is a massively more involved and technical process.
On the version of The Imperial Gatekeeper [v1.75] Procrastinatus Content Patch [v1.38] there is still a bug on Stage 4, I want to try to fix it but don't know where to start. Do I have to download Wolf RPG Maker? Then which file should I edit?