This is partly explained it Is there a functional difference between Platformio's "framework-arduinoadafruitnrf52" firmware and arduino's official firmware - #4 by maxgerhardt. The Arduino core for this board is framework-arduino-nrf52-mbedos which is actually an implementation of the Arduino core API on top of a specific mbed-os. The mbed-os part of it is still fully functional. Have a look at GitHub - arduino/ArduinoCore-nRF528x-mbedos: [Archived] Arduino core supporting mbed-enabled boards.