IMO that sentence isn't true - if it implies discontinuous response to a continuous variation of stimulus
(I was once shown a part for which a similar claim was made - and found the cause to be an internal discontinuity)
ok X-29 wing top panels preloaded against lowers would give stiffness discontinuity (not strain/strength discontinuity)
but they're imo not preloaded
yes the X-29 wing gave (in part) an unusual and useful torsional response to bending load
strong bending:torsional coupling was used in the 1930s so that propellers would have variable pitch by passive means
from anisotropic orientation of fibres or wood laminates or structural design features in isotropic material (metal)
yes such strong bending:torsional coupling could be very useful in F1 - and very hard to detect and police
this has been mentioned before