You realize that the slicer has to talk to the firmware, and it is documenting the commands their firmware implements which makes the combination of slicer + firmware print right? The whole point of the AGPL on the slicer was to prevent people from having special sauce printer firmware, get all the work of the slicer for "free" and then make their combination better than everyone else's. So you publish the code the slicer uses to talk to your device and that lets someone else implement the same codes (if they choose too).