Microbit GPIO safety
The 22MOhm resistor is a pull-up so the GPIO isn't floating. The magnitude of the resistance is significant in this instance as you wish to use the human body to ground the GPIO pin and the human body has a resistance of 5MOhm (according to the video you linked @ 3:50). If you use a 100kOhm resistor the voltage at the pin is 3.23V ( R2/(R1+R2)*V = 5M/(100k+5M)*3.3 ) and the microcontroller can't see any change.