I don't think that it's so much the Methionine nor the ingredients! If not mistaken I think it's due to their digestive tracts, cats have stomachs same as we do, their digestive systems are different ie: bacteria and germs- how it's broken down, while chickens have crops same as most all birds and most of all it's the way their bodies break it down, their bacteria and germs that they're equipped with and so forth.
It'd be like giving a cat an aspirin to me, I've always heard that aspirins would kill cats verses dogs that sometimes get them everyday of their lives. It's their systems and how it's broken down through out, bacteria and so forth. What organs can and can't handle!
Most of all it's what and how each of us have become educated pertaining to this sorts of topics. I'd prefer not to give my chickens anything else that is manufactured in this country outside of their natural grains while fruits and veggies are grown from mother earth in which is natural verses man made.
Per say a dogs bite verses a cats, cats are loaded with different bacteria and can cause an infection, same as their claws while a dogs is not as severe.
I suppose if cat food were healthy for chickens, then their grains would not be so readily available and contain mounds of corn and soy based products. Typically do we know what each and every ingredient is that makes cat, dog, chickens, goats and so on foods? Not guite, it could be code names and words for manufacturers while knowing that it has ground up body parts from all or most all other animals, the cans do not mention this!!
It'd be like fish that contains lots of omega3's in which we all need, chickens included that is healthy for them, cats, dogs and so forth, however cats, dogs and other animals are required to get a certain amount of each. It varies! Why if cat food in which contains a different mg is good for them?
This person can get by with feeding their chickens cat food while you, I or another might not be able to do such since it could possibly kill them. Maybe I'm not explaining myself well enough for you to understand my points here lol.
I'd enjoy getting in touch with a few different resarch centers books on animal feeds in order to compare them.