Most Source engine game trailers, like Half-Life 2, are “pre-rendered”. If you record a sequence of gameplay as a “demo” (kb-level file that records player movement in a level) then you can record that demo into a video at a much slower rate than the gameplay, capturing every frame; as well as add camera motions to it. There are guides for individuals to do this using the “startmovie” command.
It’s just a logical way to ensure the video is seamlessly presented, especially since framerate optimization comes late in development.