As it is often the case, I don't have the full answer but
a little piece of it. The parasites you mentioned resist the activity
of HCl-pepsin only when they are alive. If you kill them by a
comparatively mild treatment (like freezing) or damage the cuticle in
any mild way that does not hurt the rest of the parasite, they are digested
by the solution. Years ago some authors described some polypeptides
from the cuticle that were supposed to inhibit the host digestive enzymes
but I never saw prooofs of it. My pet theory (unproven so far) is that
the cuticle may not have the specific combinations of aminoacids that
are split by the mammalian digestive enzymes.
If you find some solid information, please share it.