I've seen some interpretations putting it down to luck or something of the sort, but I like to think that the dogs listened to Moist (despite not being pure-bred Lipwigzers) because of his utter confidence and certainty - which is mentioned multiple times thoughout the book in regards to people, so why shouldn't it also work on dogs? Still understandable that Moist reacts as he does though because he was relying on specific commands he expected them to be trained to obey, but in reality all he actually had to rely on was his utter confidence that it would work (which probably wouldn't have worked if he hadn't been so certain that it would work for all the wrong reasons).