He is loading only functions he is using.
Initial GL code was added at day 235. In later days few basic GL function calls were added to draw full screen quad (GL 1.x style). Adding code to load extension GL functions first happens in day 241 (for wglSwapInterval). Look for few next days after that to see how other GL function are loaded. Here's an example for much later in the code (day 370) where many different GL functions are loaded: https://hero.handmade.network/episode/code/day370/#726
Btw there is a really good search for all the days available here: https://hero.handmade.network/episode/code