Retcons are not less official, by the way.
They are if only part of the team involved in creating the original game is involved in creating the retcon, as was the case with Final Fantasy VII and its Ultimania. Sakaguchi was not involved in creating the Compilation or the Ultimania, despite his rather substantial contributions to the original game's story.

People can argue that the original game supports the interpretation of Sephiroth controlling Jenova until they're blue in the face but it won't change the fact that every single significant step taken by Sephiroth in the original game also happens to further Jenova's ambitions. Even if Sephiroth is nominally "in control", he still hasn't done anything Jenova wouldn't also have done, given the power.