He has a small hearth with a big hopper, comparatively to the burn tube area. He draws the gas off while it’s still hot.
He essentially started with a Koen style gasifier and modified it to his needs. He ultimately didn’t want the gasifier anymore because he found solar was sufficient to his expectations of powering a fridge on a boat.
I like these tighter zoned updrafts, I wonder how well they feed with an expanded hopper like that though. You’d want some thick material to hold up to the heat.