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.

They used a macro to make ‘IF’ (which is distinctly not ‘if’) map to ‘while’. So it’s really a while(1) loop, which will repeat forever, or until the program is terminated, whichever comes first.

Someone’s just being silly.