digital pins are inputs by default, so unless a pin has been explicitly set to an output, writing a HIGH value will turn on pull-ups
Right, but Brian's code enables the pull-up in pinMode(), not with digitalWrite():
pinMode( pin, INPUT_PULLUP );
Which isn't writing a value to the pin, it's presumably setting the mode to input and writing a HIGH value. That's what intrigued me.
Obviously, the Teensy does things differently so it works for him but not for Arduino. I think that syntax is nice, maybe even a little more intuitive. Writing to an input is clever but not necessarily obvious.