The two slopes conjecture for knots

Let $K$ be a knot and let $\mathcal{B}(K)$ be the finite set of boundary slopes of essential surfaces in its exterior.

Two slopes conjecture. If

$$|\mathcal{B}(K)|=2,$$

then $K$ is a torus knot.

The source presents this as a conjecture motivated by the fact that torus knots have exactly two boundary slopes; no general resolution is supplied.