But hang on a moment! Listen carefully to the man on the video. He speaks French very quickly, of course.
Got it at last. The depot is UNDER the football pitch. The earlier aerial shot shows the depot being built on the site of the pitch; the video shot shows the greensward back in place. You can even see a tram doing a "U" turn at the far end of the depot in order to get to the sidings.
But never let London transport be outdone. This is the central line depot at Shepherds Bush.