I’m not sure which post you’re referring to, but you’re right. Rabbits can also get very sick from the gram-negative bacteria in cat saliva – even just an accidental scratch from a recently licked claw can make them very sick. As much as the internet loves cat/rabbit friendships, it’s not an interaction that I suggest people allow.
I wouldn’t trust any book that says “hey, you should get another animal -which your current pet is very dangerous to- for your pet to play with”.