digging through their Instagram posts, it appears as though they've developed a newish technique for this which they call a "river joint." They match the not perfectly straight edges of adjacent boards as well as they can, then they square up and rout a tongue and groove into the edges. Where there are small gaps, they add small filler pieces as necessary, but try to minimize that.

It seems like a beautiful but laborious process that is mostly applicable to a very high effort piece like this.