The desert dwellers had guns and shot wild antelope. It's vitamin B12 that's lacking in plants but some of the other Bs are not found in meat in sufficient quantities.
IBD is an interesting subject, I have been reading about it in relation to a cat that's suffering from it. In cats, its an autoimmune disease and the treatment is to feed novel proteins or protein molecules that have been hydrolyzed and changed sufficiently to bypass the immune system targeting mechanisms but that brings me to another point: the cat is a hypercarnivore, meaning that they didn't evolve to digest anything besides meat and small amounts of grains or seeds found in their prey's stomachs. It's notable that they have a very short gut and quick food transit times, to avoid bacterial infections by pathogens associated with raw meat. Their "fibre" that aids peristalsis is the fur, feathers and bug exoskeletons of their prey.
We, on the other hand, have long intestines and colons geared towards fermentation of plant fibres and thats where we get vitamin k and others nutrients from. Our bacterial microbiome is very important in our health but we are generalists, not specialists in our nutrition. Autoimmune issues are complex and sometimes exclusion diets do help but as a general prescription, human carnivory is a nope