If Ysolda is correct that the Sleeping Tree came from Umbriel, it came from Clavicus Vile's realm.
If that's true, and it's related to the Hist Trees, can't the trees talk to the people who drink the sap? The Argonians listened to the Hist Sap during the Oblivion Crisis because it warned them of the danger.
Do you think that the Sleeping Tree will play a bigger part than what it seems?
Hopefully they will mention it again in ES6. Although I doubt it's related to the Hist, as when you consume hist sap in Oblivion it tricks you into killing innocent civilians. Sleeping tree sap has no such effect in Skyrim.
That part about the tree growing from a droplet of Umbriel just sounds like speculation to me.