Slice genus bound in $DT{S}^2$ from $s$-invariant

We prove a recent conjecture of Manolescu-Willis which states that the $s$-invariant of a knot in ${\mathbb{R}\mathbb{P}}^3$ (as defined by them) gives a lower bound on its null-homologous slice genus in the unit disk bundle of $T{S}^2$. We also conjecture a lower bound in the more general case where the slice surface is not necessarily null-homologous, and give its proof in some special cases.