It appears to me to be an attempt at a more durable tenon that could be removed hot.
Remove hot, maybe; more durable, no. Since the contact area of the prongs is very small, the wear rate is absurdly high. It's why the design was reversed to snap into the stem instead of the shank toward the end of its run---ebonite is considerably more durable than wet wood. The later prongs-into-the-stem version was indeed an improvement, but also much more difficult (and therefore time consuming) to make.
Also the design has some slop in it and might actually be easier to manufacture once jigs were made than hand fitting and finishing a normal mortise and tenon.
Actually, there can't be any slop at all. That was another problem. A bit of tobacco or etc. caught between the stem face and the shank face and the prongs wouldn't lock. I imagine no small number of lit bowls simply dropped off back in the day.
As for cutting a normal mortise and tenon, that can be done VERY quickly when machines designed for the task are used. Nothing to be gained there, I'm afraid.
My guess is that some NON shop person with political clout and/or authority within the Dunhill organization came up with the idea, and no one wanted to get in his way. (I saw plenty of that dynamic back in my corporate days.)
