I used to work as a video tech and used a SEG (special effects generator)The tapes bg is definately chroma-keyed.
The Chroma Key process is based on the Luminance key. In a luminance key, everything in the image over (or under) a set brightness level is "keyed" out and replaced by either another image, or a color from a color generator.
This process is also known as blue-screen imaging.