It’s true that many authentic rewayat regarding the sayings and martyrdom of Imam Husai (a.s) from Yadiz’s army but many narration have been reported by the family of Imam Husain and 4th Imam also. But you can say it’s a Allah (swt) wisdom. He preserves by enemies also and it’s not only in the case of Karbala but all prophets history has been reported by their own enemies too.