I'll tell you my opinion and then I'll back off. Timing is generally exceedingly important in most training tasks. However, IMO it doesn't matter what kind of timing you have IF you're scolding. You can time it just right and you're still developing a sort of conditioned reflex. Ie: Dog gets punished in the presence of other dog. The other dog becomes a predictor of a bad thing; your nervousness, your punishing, scary tone, your possibly tightening up on a leash, whatever. This is a well established phenomenon known by most behaviorists. Punishment should NEVER be used in conjunction with aggression of any kind.
I had a very reactive Doberman and went through a long process to desensatize and counter-condition him to not minding other dogs so much while on a leash. He was never perfect, but improved a lot. I've worked with clients' dogs with the same problem. It's really quite common. There is a gradual approach, using distance where the dog is kept just below his reactivity threshold. This is the general protocal. Click to Calm is a very well written out, step by step program that I too highly recommend.
Yes, IMO your trainer is wrong to suggest punishment when treating reactivity or aggression issues. Your dog may well become worse. You may see some apparent improvement as he learns to suppress his reaction in order to avoid a scolding. But as is well known by most trainers worth their weight, this is very often a short-lived remedy and you can create an explosive reaction somewhere down the road. This is not about your dog knowing you mean business. Like I said before, dogs are not that aware of their behavior in a logical, thought-out way. To your dog, you are simply being scary and it happens everytime another dog is near. He will stop behaving period because it will take loads of repitition before he may tie his own behavoir to your punishment. Meanwhile, he's developing an escalating issue with other dogs.
If you say you've tried this or that, how long did you try? This type of problem can take months and months, sometimes even longer to work through and even then, management will always be important. What did you try? What exactly was your method? Read Click to Calm if you are interested in a good, sound method of dealing with this sort of thing. Fight, by Jean Donaldson is also good. There are still others. But be careful and steer clear of anyone suggesting any repressive remedies. They do not fix the problem. They only make you think the problem is fixed. And then when you have let your guard down, your dog explodes.
Now, this is all to say if you're sure this is a dog being reactive or aggressive and not just playing. It almost sounds like you may not be recognizing it for what it is. I hope that is not the case. A certified behaviorist or good trainer....who's educated in behavior, not just how to train this and that should be able to observe and help you.