It's not really that important to the joke, since I'm pretty sure you can also replace keywords like 'if' with the preprocessor. It's just that preprocessor macros are typically (style, not syntax) ALL_CAPS_WITH_UNDERSCORES.
It's just another clue that that block isn't actually an if-block, since C is case sensitive and 'IF' wouldn't actually work.
Yeah, the attack surface of a locked-up bike is much larger than that of a bike being ridden?
If you don't want your phone stolen off the bike, a removable bike computer still makes more sense, I think.