Interesting to consider the failure mode: If the stub axle spindle failed first the bearing would not be distressed, but it is, see the photo', so it follows that the bearing failed first: Likely because the grease had dried out over the long hibernation ? However the failing bearing would have been making all sorts of distress calls before reaching the catastrophic failure displayed in the photo'. All of which begs the question of why the stub axle spindle is missing the last 1/2" or so ?
It looks as though the failing bearing started to spin on its seat some time before complete failure, but that would not in itself break the spindle, especially out on the threaded section which is clear of the seat. So, to speculate, did the spinning outer bearing, inner cone, weld itself to the inner face of the retaining washer ? This would then spin with the failing bearing 'machining' out the flat on its ID which is designed to keep the washer stationery. The washer would continue spinning, driven by the taper bearing inner cone which is now welded to it. The pinnacle nut and split pin would remain stationary and retain the wheel, for the time being. However the spinning washer is up against the stationary nut and friction would generate welding heat again. The nut would weld on to the washer and spin, shearing the split pin in its castellations.
Now this nut is on the stbd. side and as it spins will tighten down the spindle threads with a very large torque being generated on the bearing stack (on the port side it would undo the nut.) So did the threaded portion of the spindle simply fail because the nut pulled it apart ? If the spinning parts were welded, albeit perhaps briefly, there would be evidence on the mating surfaces and a sheared split pin. The forces involved would brinell the inner bearing as well.
Whatever the failure mode the lesson is careful maintenance and senses attuned to the mechanical messages your car is giving you - ideally funny noises and vibrations, pulling through the steering wheel or any other odd sensations should be investigated and resolved before complete failure occurs.
With apologies for being wise after the event
Barrie