It's difficult to call this.

I have been in a four man healer team defending the temple. We survived an onslaught of fifteen people without a death. We even survived a direct Skyshard to the face. Healers camping a flag is a very potent defense, but it's as much to do with people not knowing the correct procedure for downing a healer as healers just being invincible in a group. Unless I am interpreting your post wrong, a group of twelve would take a long time to kill a healer with other healers present, by which time they're usually dead from the non-healers lurking around.

Four healers defending a flag really is potent though. Don't underestimate just how much. They're nearly invincible without excellent coordination when attacking so that you keep all four attacked and occupied. If they can keep Stoneskin and Succor up the whole time you're hitting them they can cast with impunity and it might seem like they're not being interrupted, but of course they wouldn't be if everyone is hitting for zero.

There certainly is an element of suspicion to what you describe though. Like I said, it's hard to call. If the players are good and there's four of them I see no reason why it's not possible, but to repeat that level of performance every fight seems highly unlikely. Different opponents, different tactics. Have you encountered them more than once?

Next time, try getting someone to Braver one of them. That's a quick way of getting rid of one.