ReactOS is a full OS, so it needs to run in a VM which requires disk images. Boxedwine emulates everything it needs and can use the users file system directly. So you can just drag a setup or game that doesn't require installation right into the UI and it will work.
Yeah, I implement a good amount of the kernel syscalls that Wine needs. It was easier for me to implement the 200-300 Kernel calls, like open file, etc, than implement the entire Windows API, which is why I used Wine for this project.