Hehe, I started cracking up for some reason during the middle of the video, where the medic yells something in German right before boost the Engy up ("das vita" is what is sounded like phonetically, no idea what it means). Yeah, so I don't think I've ever seen this used in practice, so I assume it's been patched. It'd be a much faster method for Engies to get to the roof of Warpath: Point 2 than their usual method of setting a dispenser.
Oh, and I would consider this an exploit, since it can't be done with the rocket launcher or sticky bombs, which would seem far more intuitive for teammate boosting.