this video is wrong lol, i have played PLENTY of times with 2,3,4, and even solo games where the pentagon thief doesnt take anyones gun.
As long as you have a semi-competant team then its easy as hell to kill him without him taking a single persons gun (Bonfire Firesale) what i do is when we see the thief round start we all run up on the second floor of the war room (the metal grating) and whoever is the person who first sees the theif is who he is chasing, so we have that person take out their lightest gun and kite him around the circular 2nd floor area, while everyone else lights the guy up.
It helps big time if someone has the freeze gun and is shooting him with that because it DOES slow him down! As long as you guys dont take too long to drop him you are garanteed to get the achievement and the Bonfire Firesale!