I don't remember what they said in the book exactly. But I do know that food combination theory speaks of carbs and proteins separately, no matter what the source of proteins. You know, what the Diamonds said was not new; they borrowed the theory from elsewhere and threw it at the public. Useful, but not for all of us.
And you're right about animal proteins: they are much harder to digest. But some people have difficulties with soy too, and beans and lentils too. They could benefit from proper food combination.