  We consider the Hurwitz Dubrovin--Frobenius manifold structure on the space
of meromorphic functions on the Riemann sphere with exactly two poles, one
simple and one of arbitrary order. We prove that the all genera partition
function (also known as the total descendant potential) associated with this
Dubrovin--Frobenius manifold is a tau function of a rational reduction of the
Kadomtsev--Petviashvili hierarchy. This statement was conjectured by Liu,
Zhang, and Zhou. We also provide a partial enumerative meaning for this
partition function associating one particular set of times with enumeration of
rooted hypermaps.
