That is their negotiating position - the pound is a shared UK asset and they are not in a position to state anything categorically. An independent Scottish Government would clearly have a number of options but they have stated that they will be campaigning on retaining the pound and calling Osbornes bluff. Osborne is a politician , not an economist. No economist has said that it can't happen, as far as I'm aware.
From Adam Tomkins, John Millar Professor of Public Law at the University of Glasgow, who appeared as a witness before the House of Commons Scottish Affairs Committee on 15 January 2014, alongside two other academics: Prof Kenneth Armstrong (whose expertise is in EU Law) and Prof Iain McLean (who is a political scientist).
"The Scottish Government’s independence white paper appears to be have been written without regard to the distinction between institutions (on the one hand) and assets and liabilities (on the other). As a result, Scotland’s Future falls into legal error in numerous places. The following are among the key examples.
On the pound, the white paper states that “The pound is Scotland’s currency just as much as it is the rest of the UK’s” (p. 7). This is incorrect. The pound is Scotland’s currency now precisely because Scotland is part of the UK now. If Scots vote to leave the UK they will be voting to leave the UK’s institutions, including the pound. As we all know, Scotland could then seek to negotiate its way back into these institutions but the rUK would agree to this only if it was persuaded that it was in the national interest of the rUK do to so. And, as we further know, the current UK Government have indicated that it is “highly unlikely” that it would be in the interests of the rUK for it to enter a formal currency union with an independent Scotland, at least without a binding fiscal pact. This does not mean that Scotland would be unable to use the pound: any State may use the currency of another State (as Panama uses the US dollar). But for a State to make this choice means that that State has no control over its monetary policy or interest rates: rather, these matters are effectively surrendered to a foreign power".
More at http://notesfromnorthbritain.wordpress.com/2014/01/28/the-hidden-costs-of-independence/#comments