Beautiful and haunting video! And you are right to doubt the label- it's not a "mash-up," which would simultaneously use elements of two or more songs, such as using the melody line of one recording over the foundation of another. This is a medley, which merely transitions from one song to the Next (if you will). I blame "Glee" for constantly misusing the term.
Here's an example of a real mash-up:Depeche Mode vs Madonna
"What- and quit show business?" - the guy shoveling elephant shit at the circus.