Maybe the other dog owner was a total arsehole.
Or maybe they're not, maybe they're trying their best and in the moment, terrified of an accident which does tend to make people less than calm and diplomatic.
Has it occurred to you that this person maybe did not get an aggressive dog, they got a perfectly happy normal dog who, through no fault of their own, became aggressive as a result of a series of (or even a one off!) unpleasant or scary experiences?
Guess what the top way to get a reactive dog is?
Yep, have them on a lead and have other dogs (regardless of their intentions) run up to it.
The on lead dog is trapped, can't get away, can't freeze and assess the situation, can't fiddle about... the only option they have left (out of freeze, fiddle about, flight or fight)... is fight.
For some dogs that will be a bluff, all loud and flashing teeth but no substance. For other dogs it absolutely will not and as its difficult to tell, that means the dog now has to stay on a lead.
Putting them at risk of other well meaning but clueless owners like yourself who let their dogs approach and then throw smart arse remarks when told to get their dog back.
You are potentially 1 unpleasant incident away from that being your dog on the lead, yelling their head off, having to be dragged away, carefully managed etc etc.
Teach your dog that the sight of another dog is a cue to return to you - I won't explain how as you apparently know so much about dog behaviour this will be a piece of piss for you.
Don't let your dog approach others until you've asked if thats ok - owner too far away to do that, keep your dog by you.
That way, it doesn't matter if the other owner with a dog on lead is a total twunt, it won't affect you or your dog.