I'm not clear....are you saying we shouldn't use this because it may cause issues with next game version?
Clearly it just means that it works (1) only for version 0.12.5, not (2) for version 0.12.5 or higher until a fundamental rewrite (if any). For Ren'Py mods, these are the typical ways in which they interact with versions:
1. 'Universal mods'. These work with all Ren'Py games that do things 'normally'. A good example is a textbox transparency mod that can be used for almost all games to make the textbox transparent, except possibly some that already have a built-in transparency glider, and with the caveat that in some games readability with a transparent textbox is very poor.
2. Mods that work for all versions of a specific game, so long as it is only extended, with no fundamental changes to how it is implemented. A good example for that is enabling rollback in games in which it has been disabled for no good reason.
3. Mods that need to be updated regularly to preserve their full functionality, but can be used with a newer version of the game without breaking anything. A good example for this is a mod that adds a gallery to the game. This can often be done like the second category of mods, except that obviously when the game updates, the mod should be updated to include scenes from the new content.
4. A mod that is incompatible with all versions of the game other than the one it was published for. A lot of walkthrough mods are of this type, as they replace the files that contain the core game logic. When the game is extended, these files are updated to either contain the new content or link to new files with the new content. When using such a mod with a newer version of the game, if you are lucky, the
old content is playable without issues. But you can't reach the new content.
As this is a walkthrough mod, it is likely of the fourth type. I.e., the first two variable version numbers of the mod and of the game must always be identical.
Occasionally (not here) there are also additional patch numbers for bug fixes. They don't have to agree. E.g. here, mod version 0.12.5 is compatible with game version 0.12.5, but not with 0.12.4 or 0.12.6. If at some point a bugfixed mod version 0.12.5.1 comes out, or a bugfixed game version 0.12.5.1, then you can still combine this with game version 0.12.5 or with mod version 0.12.5, respectively.