Even with a "stiff" SLE it just couldn't handle the load....
Stiffness is the ability to resist deflection under load, not the capacity to withstand it. A stiff spar won't necessarily be bent very far before it catastrophically fails.
The centre section of a Flexi. needs to be both stiff (to remain flat) and strong (to not explode). What it needs to be is SOLID
carbon fibre rod, with as few ferrules (ie; weak points) as possible). I bent the thick brass ferrules on a Super 10 by not pushing them all the way in and they were beyond tough.
The old Hyperlight spar sets that Flexifoil made to ULificate their foils (yeah, it's a word) used tubes in the centre section and had a max. wind speed just slightly above their minimum.