That debt would only be repayable when the rUK repays it. That does not mean when the rUK rolls it over as part of its debt management programme: that means when it actually in net terms repays it, and until then Scotland cannot be expected to make payment towards something the rUK is not doing;
So, rUK follows a different economic policy than Scotland. That means debt management will be different. But if rUK repays debt then so must Scotland. Which subjects Bawbie money supply to rUK decisions.
Note this also works the other way around. Say that Scotland does better than rUK. But it’s not allowed to pay off that debt……actually, the very fact that Snippa’s not even thought of that shows how unlikely he think that is.