Cats do not have the long digestive tracts of ruminants to efficiently ferment raw starch or fiber, but that doesn’t mean they cannot digest plant matter at all. It also does not mean they are unable to obtain any nutrients from plants or from fiber.
I’m pretty sure cats do not have a strong enough digestive system for a plant based diet. I’d be interesting to see what they fed them
https://rawfedandnerdy.com/what-is-an-obligate-carnivore