I think you did right by not reacting. If you had it might make Tali anxious with other big dogs in the future, especially if he's on lead and they're off lead. It's never easy for the on lead dog in that sort of situation, but sometimes it's unavoidable. If Louis is on lead I know that he's more reactive than off (I'm certainly not saying that Tali is) but when they're on lead they can feel more vulnerable can't they.
Overall it sounds like Tali pushed his luck a little and got told off, but nothing more than that as Tali didn't retaliate and the other 2 dogs didn't follow up so it didn't escalate.
My issue with it would be that I feel quite strongly that people on horseback shouldn't have dogs running loose with them. There's no way that they can have proper control of their dogs. I used to own horses and ride but I wouldn't dream of taking dogs out with me while riding. What does the rider do with her horse if her dogs are in, or cause, an emergency situation?
A friend of mine used to do this many years ago. Her Border Terrier attacked a small dog one day and did a lot of damage to it. The other dog owner couldn't stop what was happening and neither could my friend as she was on her horse. All she could do was try to call her dog off and pay the other owner's vet bill afterwards. More recently I heard of a dog killed on a road while it's owner was riding her horse. She thought it would be safe as it was a quiet country lane.
Sorry, diverted your topic a bit Mel, but IMO the horse riders were pretty irresponsible.