Yes, currently the pyd is only for Windows. This project is a trial run to see what the response to it is. If The Game Creators, the makers of AppGameKit (I am not affiliated with them), decide to continue the project beyond its initial 12-month trial, it will be up to them to decide what they want to support.
In my own opinion, Linux and Mac OS support should be doable. Their library supports both of those OS already plus Android. I don't think Android would be as easy to do as the others, but I haven't looked into it much and don't know for certain. The main reason for the current limitation is that Windows support is the easiest for me to do personally.