Had not heard about that one.
Yeah, it's a glitch. Here's the explaination/how it's done.
[spoiler]
Normally when you start the game and the zombies are let into the mall, you race to the stairs. On the way back into the mall as you're just about to leave the warehouse and enter Paradise Plaza, you have a cutscene with Jessie, setting up the first mission to help Brad with Carlito in the food court.
There seems to be some function in the game that is either time-based or zombie-kill-based. When you cross this threshold, the next time you go to the warehouse, you get the cutscene about the queen wasp. So what you do is fight the zombies in the entrance plaza before going up the stairs. Again, it is not known whether it is time-based or kill-based, but if you kill 600 zombies in the entrance plaza before going up the stairs, you will trigger the queen wasp cutscene the
first time you go through the warehouse instead of your second or third time.
Subsequently, Jessie's cutscene does
not take place and you are never given the mission to help Brad, nor any other of the important missions. Otis still calls you about the side missions. Actually it's a bit maddening because it took me about 13 game hours to kill 600 zombies so he had quite a queue of missions to call me about by the time I actually got the transceiver. I was going for zombie genocider so I wasn't answering him and after awhile I was like, "Jesus Otis, can't you fucking assume I'm dead by now and stop calling?"
Killing the 600 zombies isn't as hard as it sounds. I used the hammer throw to take out big rows of zombies. It quickly thins out and you make your way to the entrance. After you clear that and turn around, a ton more spawned behind you and you can go back and kill them. Then you repeat this, but each time you make your way from the back to the front, less and less spawn. By the end, when I turned around there would only be 2 or 3. And most of them have hunks of meat they are fucking with so it's really easy to just run up behind them and do a face smash or suplex to kill them.
[/spoiler]