In my opinion the Zombies in the video didn't seem to be spawning in front of you, but were in fact coming out of hatches, other rooms or vents. There seemed to be warning signs, like clouds of dust from the ceiling of the elevator for example.
Plus the Zombies are attracted by noise so using the standard pistol or carbine, results in more Zombies joining the fight. Hence the silenced pistol.crossbow being available. I've only played the demo but keeping quiet did seem important. I died a lot when I tried to start shooting as all the nearby Zombies would appear, but I could snipe them individually with silenced pistol or crossbow.
I believe the mine exploded as it was possibly placed to close to a dancing Zombie, or possibly Patrick moved too fast after laying the mine, he seemed to be running backwards and pulling the gun before the mine was laid properly. It can be thrown and this may have been what happened due to running backwards mid lay.
Dealing with the dancing Zombies instead of sneaking past was probably a good idea as the music and disco lights would possibly have stopped at the worst possible moment. Also my guess is if you kill the DJ the disco stops and you have to deal with the remaining Zombies. The Zombies are only ignoring you and the flare because of the lights and music .
When I buy the game I will try to remember to check out some different strategies in the Zombie disco. :)