Juncker doesn't get final say on extension. It would be up to the EU Council. So they could still firm up on the This Deal or No Deal stance or they could torpedo it and the Deal by indicating they would offer an Extension or even equivocating.
However looks like EU really want WA Done and Dusted before new session starts properly in November. That way they can move forward with the UK negotiating from the outside - probably suits them better even if the long term aim is / were still to persuade us to rejoin via 2nd Ref.