I've never had a dog, so I can't comment on how I'd feel, but presuming it feels similar to seeing a small kid kicked, I'm impressed by his calmess, going to pick up the dog and presumably calling the police. In that scenario he was justifed decking them - purely as a deterrent, so they don't go around doing it again, not for revenge or honour. However, that would be more time before he can attend to his dog, plus there's a chance they fight back well enough that it really delays him. Honestly he probs made the right choice to attend to his dog, stay calm and maximise the chance of the police and courts fully taking his side. He dealt with it, he didn't just back down. The problem is if the CJ system doesn't do its job, and then citizens have to use their own deterrence.
It's also possible he decks them both and then someone from their side comes to attack him, his family, friends or dogs in the future, which isn't worth it, from a pragmatic standpoint.