I've seen a lot of these "source engine animations" over the years, but the ones this person makes are far and away superior to the vast majority of them. Very well made!
That was actually made using Source Film Maker [hence the "SFM" in the title], which is a dedicated Source engine movie making program. Before that, people just had to set things up and film it in Garry's Mod, which took a lot longer to get anything usable and wasn't nearly as versatile or complex.