The magisterium’s usefulness with respect to interpreting Scripture isn’t based on how many definitive interpretations of individual biblical texts it has rendered. Anytime the Church rejects a heresy or defines a doctrine, it’s interpreting the biblical message, even if the sense of no particular passage is defined.
For example, the Jehovah’s Witnesses interpretation of John 1:1, Colossians 1:15, and Revelation 3:14 (that Jesus is a mere creature and not God himself) was ruled out by the Catholic Church when it defined the divinity of Christ and existence of the Trinity (at the Councils of Nicaea and Constantinople in the fourth century). This remains so despite the lack of an ex cathedra definition by a pope or a conciliar decree precisely defining the sense of each of these passages. The Church has the power to teach infallibly the overall, doctrinal sense of biblical revelation as well as to interpret particular verses.