It is essentially a batch box without the precise dimensional configuration which makes a batch box efficient/effective. They may have lucked out and gotten close enough for it to work well enough. Building one of these as an 8" or even 9" or 10" batch box could be an optimal solution. The bigger sizes may be able to support a down-and-back layout with the chimney next to the core for better draft. This could allow a full load of wood which would burn seriously hot and heat up the huge mass well. I would go with a masonry bell with access plate instead of the steel drum, so as not to give off too much instant radiated heat.