So for instance MAX_PLAYERS is 512.
It checks it, gets the int 512.
Int 512 decides if the player is online?

What are you talking about?
It looks for the player of choice in a loop (from 0 to MAX_PLAYERS) max players being whatever you set it to. in your case 512 players? anyways..
the player being connected and set to a slot in the players[] array depends if the player is logged on.. lol
Just ignore him, let him think hes right.