Automorphisms with quasi-discrete spectrum, multiplicative functions and average orthogonality along short intervals

We show that Sarnak's conjecture on M\"obius disjointness holds in every uniquely ergodic modelof a quasi-discrete spectrum automorphism. A consequence of this result is that, for each non constant polynomial $P\in\R[x]$ with irrational leading coefficient and for each multiplicative function $\bnu:\N\to\C$, $|\bnu|\leq1$, we have$$\frac{1}{M} \sum\_{M\le m\textless{}2M} \frac{1}{H} \left| \sum\_{m\le n \textless{} m+H} e^{2\pi iP(n)}\bnu(n) \right|\longrightarrow 0$$ as $M\to\infty$, $H\to\infty$, $H/M\to 0$.