True allergies are not the same as intolerances, which generally involves an inability to digest a food or a component of the food. Allergies are immune system reactions. Certainly, you could get some digestive issues with an allergy, but that is not generally the major symptom.
And, it puzzles me that cat food would have grains in them (not that I don’t believe you, I’m sure they do it to save money). Cats are true carnivours and cannot live without meat, unlike dogs which can (altho I don’t recommend it).
It’s basically impossible to make dry cat food hold together in pellets with significant carbohydrate content. And this cat refuses to eat wet food (or any food other than dry cat food).
The line between allergy and intolerance is very fuzzy. The inflammation resulting from “intolerance” is caused by many of the same immune system cells and molecules that are responsible for a direct allergic reaction. And the inflammation in the digestive tract is what results in the digestive problems and in severe cases, in serious nutritional deficiency due to the inability of the inflamed digestive tract to absorb nutrients from food. The inflammation can be caused either by a direct allergic response to certain substances or by the failure to digest certain substances which then reach parts of the digestive tract that weren’t designed to handle them.