Associativity conjecture for tensor products in shifted category O

Let $V_1,V_2,V_3$ be objects of the shifted category $\mathcal{O}_{sh}$. Associativity conjecture. There exists an isomorphism

$$V_1\otimes(V_2\otimes V_3)\cong (V_1\otimes V_2)\otimes V_3.$$

Thus, $k$-fold tensor products in $\mathcal{O}_{sh}$ are independent of the choice of parenthesization, up to possibly non-canonical isomorphism. Associativity is needed for the tensor product to define a monoidal categorification; the paper explicitly assumes this widely believed conjecture, but no proof or resolution is supplied here.