I am so so sorry for your loss. . .
Here's what I know about EC.
The thing about EC is that most rabbits from either not good breeders or shelters (and even some GREAT breeders!) have it. In fact, I can ALMOST guarantee both mine do (one was in a hording cruelty case and so I'm sure the other has it just by proximity- it is transmitted through the urine and I know he's been around her urine). But with EC, a rabbit with a normal and healthy immune system shouldn't have symptoms (I think some have a genetic predisposition to a weaker immunity against parasites, including EC, so some otherwise healthy rabbits can have it, although this is more rare). Because your rabbit was 11, he might have had issues such as renal disease, liver disease, just a lack of immunity due to age (note none of these are preventable or indicative of being a bad owner or not noticing anything- these things happen because they are alive and using those organs constantly). These (and many other things) can reduce immunity to the point where a latent infection of EC then 'flares up' and can lead to death.
Now, technically the vet cannot say yes it was the EC, because it could have been something else and he just tests positive for EC antibodies. It really could have been something else completely unrelated. For example my last rabbit who passed had EC, tested positive, but died due to septicemia from a perforated gut (basically bad luck- perforated guts often happen without any symptoms/complications in all species) in combination with chronic kidney disease. So the chronic kidney disease (and age) lowered his immune system enough that EC and septicemia could have killed him, it just happened to be the septicemia which ultimately lead to his death.