What you see is gameplay of Left 4 Dead back when it was a simple project using Counter-Strike: Condition Zero. Yes, this is older than zombie_city and the leaked L4D footage that was re-discovered a few weeks ago, both of which were based on Counter-Strike: Source. Zombie_city was originally built for CZ and ported to CSS, but we never got any actual content from the CZ era of L4D until now.
Where did the code for this come from? On November 22nd, Valve updated Counter-Strike 1.6 so that it'd work with the 25th Half-Life anniversary update. This update also briefly accidentally introduced code for a very, very early Left 4 Dead prototype, back when it was a mod for Counter-Strike: Condition Zero. The code was removed, but not before fans snagged 1.6's dll files and worked some magic to make a quick "mod" built on Condition Zero. The Terror-Strike code was included because 1.6 has code from CZ in it, and it seems someone forgot to strip the Terror-Strike code from the dlls before originally shipping the update.
Also, it seems that this prototype includes fog in certain maps, such as de_inferno_cz.
Gameplay mechanics and other interesting things highlights:
0:00 T starts with USP instead of Glock 18.
0:33 Player death sequence
1:10 Screams from HL1 playing after planting bomb
2:36 CT "zombie" defuses bomb
4:49 Bomb does not detonate after being planted. All CTs dead.
5:00 Start of regular gameplay
6:02 T victory