Plus they are too stupid to do DNA tests.
Even if, presumably, Cullah has been told the baby is his his men will have known all this time that the baby is not Kazon. They've known that for months as a fact so Cullah's sudden announcement that a human hologram on the side of their prey has, in the midst of sabotaging their glorious agenda, announced that the baby is Cullah's will mean NOTHING. Why should they believe that? Cullah will be nagged by his grudging respect of Seska's intelligence to believe it if she did but that baby will go back Kazon society with a huge cloud of doubt hanging over its head.