I think your trainer is right if you don't have a reliable recall, don't let your dog off a long line in a public place. All your dog is learning to do is ignore your call, and there is nothing you can do about it.
However a long line should be used as a training aid, not just to pull your dog back on, because you are right under these circumstances the dog is also learning nothing.
The line should be allowed to trail on the floor, just before you call your dog, pick up the line, as you call the dog if it does not look at you, move the line from side to side, to get the dog to look then carry on with your normal recall, if the dog doesn't look at you, you will need to do it harder to get attension.

Alos be honest with yourself, how quickly does he come without other dogs there, for instance if he is sniffing something.

Good luck with your training.