The part of plants ferrets can't digest is the cellulose in the plant cell walls. Many herbivores can't digest this either, which is why their stomachs often contain bacteria that can. Protein is protein. Unless the plant protein is still in the plant cell walls the ferrets won't have a problem with it. Plant protein doesn't contain the full range of essential amino acids though, and that might be a problem if all the protein is plant derived.
Been feeding Chudleys (and TPP which is the same shape) for years now, never had a problem.