  Recently Ibukiyama proves an explicit formula for the number of certain
non-principal polarizations on a superspecial abelian surface, extending his
earlier work with Katsura for principal polarizations [Compos. Math. 1994]. As
a consequence of Ibukiyama's formula, there exists a geometrically irreducible
component of the Siegel supersingular locus which is defined over the prime
finite field. In this note we give a direct proof of this result.
