I just did a fresh install of Arduino 1.8.3 on Windows,
I checked out the Arduino-Boards repo as Documents/Arduino/hardware/avr/keyboardio
I ran “make” inside that directory to checkout git submodules
I checked out Model01-Firmware into Documents/Arduino/Model01-Firmware
I opened Arduino and set the board to “Keyboardio Model 01”
I opened Model01-Firmware.ino and built it, without any errors.
Can you give a process like that a shot?