Ekedahl-Oort strata with a superspecial factor in unitary Shimura varieties

Let $\mathbb{k}$ be an algebraically closed field of characteristic $p>2$ and let $O_K$ be an imaginary quadratic order in which $p$ is inert. For any $a,b\in \mathbb{Z}_{\geq 0}$ not both 0, denote by $M(a,b)$ the unitary Shimura variety that parametrizes principally polarized abelian varieties ove