Warren and Jeff, the draw-bored mortise and tenon is definitely the ideal joint for this in my opinion.
Alex, I suspect it’s mainly because the parts are thicker. Joint testing to failure only shows what is strongest in that respect, it doesn’t show what lasts best over time in normal use. As example Shoji use a double tenon, traditionally. Small double tenons are not terribly strong joints in ultimate breaking strength but they hold very well over time with traditional glues.
Bumbling forward into the unknown.