I wouldn't be so sure about that. A lot has changed since that leak. Before the introduction of the bosstiary, I checked the rare bosses and many times I had a situation to meet the monsters alone without a boss. For example, the War Wolves from the Big Bad One raid or the Banshee from the Dibilis the Fair raid. At the same time the Boss was not killed but puffed. So some monsters definitely exist longer than their bosses. I just don't know if they live until the end of the server save because that would have to be checked somehow, and I haven't researched that. However, I suspect that this type of monsters does not disappear.