This run uses two exploits to make the playthrough easier: a well-known glitch called the "Jesus room" that exploits the infected's ability to find of your location, and medkit exploit that abuses The Director's knowledge of your physical condition.
A brief summary of the Jesus room from GameFAQs:
"The Jesus room" is the name commonly used for the room under the satellite, on the rooftop finale of the No Mercy campaign. Once inside this room there is a glitch that makes it so no zombies can come in whether common, special, or tank. Eventually if you wait long enough both hordes of zombies will completely die, along with the two tanks prior to the helicopter's arrival...."
You can read how to access "The Jesus Room" on the Left 4 Dead wikia. Once your character is this room inside both common infected and special infect will be unable to reach you as the game believes the room is inaccessible. While the glitch was in frequent use zombies would not attempt to breach the room, and instead would turn back. Their programming would view the door in a similar way they would normally view a wall - inaccessible and cannot be broken. After a period of inactivity they would simply die off. However, the Jesus Room has since been patched out of the Rooftop Finale level and is no longer accessible.
The second, and less well-known glitch is the medkit exploit. Almost everything in Left 4 Dead is controlled by an enigmatic AI called The Director. The Director controls the quality of weapon spawns, the frequency of special zombie spawns and even the layout of some maps. The Director is capable of grace and pity in addition to it's default malice. Cooperative teams will be rewarded with more items and fewer zombie spawns when The Director see frequent assists and team members healing each other.
To exploit this glitch the speedrunner is almost continuously healing themself. Because of the constant healing The Director assumes they are in a bad physical state. The Director will soften the difficulty when players are hurting and spawn fewer zombies. In this case, now that the speedrunner has falsely convinced The Director that he is in a critical condition the game will take pity and spawn fewer zombies to harass him - making the final escape easier. To my knowledge this glitch has yet to be patched out
Both of these exploits in conjunction severely limit the number of zombies that spawn and make what would normally be a very difficult finale appear trivial.