Actually he is using the collar to make a behavior extinct, so there wasnt anything necessarily incorrect about his approach if he does not want to use the collar for anything else. Ofcourse there are associated risks with getting down and dirty with the collar right away, superstisious associations for one. However, in this case the dogs focus was intent enough on the cat that Id be comfortable saying he associated the stim with the cat.
I see a prey driven dog that wants to eat the cat. End result is the dog will likely associate cat with stim if he does another similar session or two and avoid it. Similar to snake avoidance training but worse since the dog has a lot of intensity for the kitty.
Welcome to the world of dog training not all cases are pretty or easy to handle. Some of them look quite messy during the process due to the severity of the behaviors involved. A classic case of short term pain long term gain.
You cant throw a treat and a clicker at a case like that imo.
If you think that short clip was messy try cleaning up whats left of your pet cat after its been chewed on by a GSD.
Such shy animals are in all circumstances an encumbrance to their owner, who must be ashamed of such a dog, and a disgrace to their race. Under no circumstances whatever must they be used for breeding, however noble and striking they may appear.