A more likely explanation is cat's kittens died and she chose to nurse the puppies, which is the kind of thing that happens all the time.
There's a cat in that pic?........oh yeah, now I see it!.......
I have had kittens nurse off my mama dog many times. I have also been told of cat rabbit mixes. People where I camp swear they have seen them but I never have and I think it highly unlikely cats and rabbits breed or cats and dogs breed either. Sounds like BS to me.