I'm not sure this would work. The "game" is running in a different process than notepad and WM_SETTEXT accepts a memory pointer as input. This pointer would be meaningless because it's coming from a different process.

Windows does automatic parameter marshaling for system messages (0 to WM_USER-1), which WM_SETTEXT is a part of. Since the meaning of the parameters is well-established, the system can allocate memory in the target process and replace the pointers with meaningful values for the target wndproc.

You can also allocate memory on the hooked process, Notepad in this case, and modify that one then use WM_SETTEXT. I thought about that when I saw his "wait to fill Notepad with WM_CHAR" which took like 10 seconds? Anyway WM_SETTEXT would be faster then 30 FPS he achieved with accessing Notepad's buffer memory since he's doing a lot of extra-work there to achieve the final result.