but the Arduino implementation is preset to a default pinout, so you just need to look it up, and hook up MOSI to MOSI and so on. As of now, it only uses one of the ESP32’s two free SPI units.

Plus the Arduino needs to be plugged into the host computer via USB to function, a rather quaint idea to many in 2018. These were just some of the reasons he decided to port Grbl to the ESP32 board.