I don't get what you're trying to prove here. You've overplayed a short game, does that disprove his point?
In terms of content, L4D has nothing on Half-Life 2. It's a mini-game in comparison.
Half Life 2 has about 12 hours of cinematic shooting with a good plot and memorable characters, and nothing else.
Left 4 Dead 2 has countless hours of playing two games' worth of campaigns with a larger cast of characters and several game modes including a massive competitive multiplayer scene, of which I have been a part of for years. That's not including the co-op campaigns, survival modes, custom servers, mutations that have increased over the years, workshop support for campaigns and mods, and so on.
This doesn't even include the animation, modeling, and other technical aspects that were improved dramatically for Left 4 Dead 2's release.
So, no. It's not me overplaying a short game. It requires a little thinking to realize that it's far from a mini-game or sideshow.