Sharing this for myself and anyone else in this situation:

• Install libfuse2: sudo apt install libfuse2
• Install qt. If you try to the old package qt5-default that every search result says to use, it won't work, the package doesn't exist anymore. Instead run sudo apt install -y qtcreator qtbase5-dev qt5-qmake cmake.
• Now the weirdest one, that I think is specific to this keyboard: I was getting a PermissionDenied error when trying to add the Polyglot keyboard. The fix I did was to add myself to the dialout group so that it could talk to serial devices??? sudo usermod -a -G dialout `whoami, log out, log in. Yes, I tried running the app with sudo. The app threw a stack trace and would not boot.

I did not expect this since AppImages are described as being self-contained.