Izuna mastered his Sharingan at a very young age, alongside Madara. Izuna's Mangekyō Sharingan, that developed some time after Madara's, took the appearance of three bars stretching out of his pupil. When Madara took his eyes, the two's Mangekyō combined. Izuna's loss of his eyes rendered him blind, but he managed to survive, and continued to battle. This sudden disability however eventually led to his death.
While I agree it could just be false info; Wiki states twice Izuna died in battle due to his blindness.
He says he awoken the Rinnegan before his death just means his eyes mutated into them from the Sharigan sometime before he died. Muu can split his body without the Rinnegan so it wouldn't be hard to say maybe Madara could do it too.