That's exactly the point - 'untrained or badly trained'. This means that one has to devote endless amounts of time to training this dog, as well as being constantly engaged with it. The cat, on the other hand, does not lend itself particularly well to training. It doesn't need it either.
You are so right about adopting grown dogs. Things can turn out well, but there is always a danger, a huge danger that something bad will happen because of things from the past.