I can see how one or two 9 volt batteries, in a full on short could create enough heat to ignite some sort of fuel. What I don't understand about the video, is how that small ignition could have propagated into a whole house fire. I can only assume that the batteries in question were in very close proximity to a fair amount of fuel, perhaps paper etc. IMO, there was a second mistake: throwing out the batteries into s source of fuel. In our community, we have a separate recycling bin for paper, cardboard etc. A great fuel source if you put the batteries into it together in a paper bag. Note that the recycling bin has an attached cover; closing it might be a useful safety precaution also.