Hi Richard,
I don't want to bust your balls about this, since you are a very rare thing- a witness who knows about aircraft and who filmed it. The devil is in the detail here. How far do you reckon he was from you when you hear the engine quit in the video? If he was say about 300 metres away from you, you have to wind back the video pretty much a full second. That is where the engine actually quit.
I don't know enough about this configuration of biplane. Could it be that trimmed for takeoff under full power, it would be inclined to pitch up with an engine failure? Or maybe when the engine has quit, he has pulled back to gain height and then pitch down for glide?