The idea is simple. Games takes many man hours to make. So why not sell almost finished games for the user to fix. This way the user does’t need to make a game from scratch. They way I imagine it could work is if the problems left undone in the half finished game are intentional. The aim is then that the user will get the game to work with tutorials and thereby learn a lot along the way.
This idea can also be made to fit the users experience if you make the game modular. Here you first you fix the minimum requirement of the game. The bare game will then be able to run but different levels of the game are left out. Those will require more of you as a programmer when developing the missing parts.
Even big 3D shooters can be made as an educational project. Here you don’t need to compile the software. Instead an autograder compares the code written by the user with the working source code. If it passes the tests then that part of the game is unlocked.