A blind version of a through wedged tenon that fox wedge joint is.  That is one that is hard, if not impossible to duplicate using a CNC.  The blind nature of it hides how clever it is.  I'm guessing that it became more widely known about after some original furniture made by a clever craftsman finally came apart.  If the glue eventually fails/turns to powder it can still come apart.  Hard to make an undercut tapered hole for a round tenon. I made a table for a PC that I knew would be rolled from my office to a classroom and back.  Normal mortises and tenons but pinned in place with a little dowel drilled though the side. Never came apart or loose while in use.  I eventually brought that table home with the plan to salvage/recycle the wood in it.  Had to cut it apart as I couldn't get the joints to back out even after drilling out the dowels. I'd tapped them into a slightly smaller hole in the tenon which split/spread the tenon inside the mortise. 

4D 

some observations of the original picture.

 

the legs splay out, so any downward force (you sitting on it) results in the wood trying to pull out of the central shaft. and the more it is sat on, and the heavier they are, the greater the force.

 

when you build something like a deck, with large amounts of downward force (weight of the deck itself, weight of the BBQ party on top), we know not to put connections in shear, like supporting all the weight on a nail or bolt.  no, we design things to transfer the load thru the massive wood parts, so it stays together.  sadly, sometimes builders miss that point, and you get deck failure and people hurt or killed.