Most of the relevant work seems to indicate that the nutrients found specifically in vegetables/fruits aren't digestible by cats, and the nutrients that they need are found in animal tissue (usually muscle or fatty tissue). So the inclusion of these ingredients in their food is not poisonous, but adds indigestible mass that they don't need.
For example, in The carnivore connection to nutrition in cats (JAVMA, Vol 221, No. 11, December 1, 2002) Dr. Debra L. Zoran states:
Vitamin A is found naturally only in animal tissues, and it must be
provided as the biologically active form in diets formulated for cats
because of the fact that cats cannot convert β-carotene (which is
plentiful in plants) to retinol (the active form of vitamin A); this
conversion is not possible, because cats lack the necessary intestinal
In high enough levels, being unable to digest some materials can actually be harmful to the cat. For example, benzoic acid is one of the components in cranberries (a popular fruit to add to cat food because of its folklore status as a cure for urinary tract infections in humans). Too much benzoic acid can be fatal for cats.
Outbreaks of poisoning affecting 28 cats have followed ingestion
of meat containing 2.39% benzoic acid. The effects were nervousness,
excitability, and loss of balance and vision. Convulsions occurred and
17 cats either died or were killed. Autopsies showed damage to
intestinal mucosa and liver. The sensitivity of the cat may be due to
its failure to form benzoyl glucuronide and toxicity may develop with
quantities greater than 0.45 g/kg single doses or 0.2 g/kg repeated
doses (Bedford & Clarke, 1971).
However, you'd have to feed your cat a LOT of cranberries to get to this level of toxicity, so I don't believe your cat would be in any danger from a commercially produced food that included cranberry.
Cranberries are somewhat of an unusual case. Their inclusion is often to promote urinary health, not just to increase the "nutritiousness" of the food. Skepvet summaries his findings:
There is weak theoretical justification for using cranberry products
for UTIs, though none of the supporting preclinical evidence involves
dogs or cats. There is conflicting clinical trial evidence in humans,
and no clinical studies in dogs and cats. There are weak theoretical
Peas and pumpkin can also be included not for nutritional value, but to bulk up the stool and relieve constipation. I don't generally recommend feeding fiber to an otherwise healthy cat and prefer to monitor the situation myself. GI symptoms can be the first sign of an illness, so I prefer to know as soon as possible that something may be wrong.