In the Oblivion Crisis we see the Hist and the Argonians being able to supercede Mehrunes Dagon within his in own realm. Perhaps the Hist are originate on a higher subgradient than the et'ada, perhaps the original Hist trees were of Anu's world that he dreamnt of but being that they are trees have been bystanders since the beginning but also have been blessed with god like power just for the sake of being part of the dream since the beginning.

But again being that they are trees have gone un noticed and have passed through the gradients with ease but with the downside that they can only exert their all knowing influence on those who interact with them directly. Ingesting Hist-Sap is actually experiencing all of history and being as close to the beginning as possible all in one instance. Those who haven't lived their whole life with the Hist and have always drank Hist-Sap cannot fathom the brevity of it causing hallucinations. Argonians experience everything at once and have no perception of time because of their connection with the Hist who themselves exist beyond time.