Several sources such as Trevor Noah claim that Zulu leather shields could function as armor against muskets after being treated with water. How accurate is this?
I can't speak to the musket-proof nature of the Zulu shield, however I have read that shields constructed of similar materials having that property.
In "Empire of the Summer Moon: Quanah Parker and the Rise and Fall of the Commanches, the Most Powerful Tribe in American History" author S.C. Gwynne points to buffalo hide shields offering great protection from Muskets.
"As before, mounted warriors encircled them, firing arrows and using their thick, buffalo hide shields to deflect bullets (which they did quite effectively)." Page 98.
"The Plains Indians' shields, made of thick, layered hide, were surprisingly effective against bullets, and at the right angle could stop any bullet from a musket and even, later, a rifle." Page 133
"Why had the Indians made a point of taking the bible? According to Goodnight, Commanche shields, made from two layers of the toughest rawhide from the neck of a buffalo and hardened in fire, were almost invulnerable to bullets when stuffed with paper." Page 175
So although I do not possess any direct knowledge of the water treatment, or the Zulu shield in particular, the Commanche example makes Noah's claim credible in my view. I don't think there is too great a difference between buffalo hide and that of Nguni cattle that were the principle material used by Zulu warriors in shield construction.