This achievement description was tweaked for PC since splitscreen is currently not possible. On Steam it now reads as: "Halo: CE: Play a CTF custom game on Blood Gulch with 4 players. Tri-tip optional."
So just play an online custom game with 3 other people on Blood Gulch and you should be good.
Credit to this thread:
https://steamcommunity.com/app/976730/discussions/0/17445139...