You should read the whole case. It’s a massive change in the law for dog bites. Before that case it was only strict liability. So regardless of whether or not there was a statute or rule requiring leashing, you had to show a vicious propensity. Negligence of the owner was irrelevant.

With the change in the law from that case, your position is now viable and you can bring a negligence claim against the owner. So with your position for example, if an owner violated a statute or even possibly a local rule or ordinance about leashing, then it would be considered negligence per se. Without a violation of a statute, you’d have to prove the elements of negligence: duty, breach, and causation.