This footage is taken from something called Half-Life Alpha, a demo of Valve's classic shooter playable in 1997, a year before the game would eventually be released.
As you'll quickly see, most of this is actually a tech demo. This might explain why there's a Macross Pod hiding in a stairwell.
Update: Ah, the video was captured from a leak of the demo itself. Want to try it out yourself? Have at it.