Mustum is not “doubtful matter”…. While it is illicit to use the freshly pressed juice of grapes for Mass, it is not doubtful.
Only if you believe Ratzinger's 1994 decision. Prior to that time, the question was disputed among theologians and therefore considered positively doubtful. Ratzinger's 1994 decision holds little weight for me. This is the same Vatican that also said, in 2001, that the Assyrian Orthodox liturgy that lacked a consecration was valid, the same Vatican that canonized Roncalli, Montini, and Wojtyla.