I'm a little confused. If the weak link is less than 200% of the max
AUW of the glider, how is is possible to generate ~3.5G?

During a winch launch, the load from the winch line is concentrated at
the wing root. It is not distributed along the wing span as normal
flight loads are. While the pilot may be experiencing something around 1
g during a winch launch, the wing roots may be "feeling" the equivalent
of high g loads.