I am almost certain that the video you mentioned is by Frank Klausz (one of my favorites) but it has been a long time since I watched it. Without watching it again, I don’t remember for a fact what method was used at the box corners but I seem to think that very dry soft wood was used with tight dados or sliding dovetails and the side pieces were allowed to run long past the joints. The sides and ends were planed flat on the bottom edge after assembly to help ensure a tight seal with the bottom before the groove was established by compression using a bent nail or wire and a hammer. Then the box was planned down again (just to the bottom of the compression groove). The flattened bottom was nailed on very tightly. When the water hits the wood the wood swells with the compressed groove swelling more than the surrounding area. The swelled, previously compressed wood, forms a gasket.