It was indeed a secret that no one knew about the Sif.
They were quite reserved and enjoyed simple lives without much conflict – at least post the Grand Wars. They cherished these simple lives; adoring the blessings of the seas, the fruits of the forest and the life that both gave.
The Luminants had also been enthralled by the peace when they were forcefully brought to Opungale. They had assimilated to the burdenless life much better than they would if they had been brought anywhere else on Aigas.
However, reserved and kind as the Sif might have been, they were not helpless plant worshippers as most thought.
They had a secret.
The Sif did not have unnaturally long lives… naturally. They were similar to humans, though, dietary differences did give them an edge life expectancy wise.