I found a bit more information about the Mwinyi Mkuu. It turns out that the caption of the picture was wrong - the people depicted were not from the 15th century:
"Mwinyi Mkuu, the 'Great Owner', last descendant of the Shirazi sultans who preceded the Omani Sultanate, together with his son. He died in 1865 and his son soon thereafter."
https://zanzibarhistory.org/Palatial_Zanzibar.htm
I'd be interested to know if there are any photographs of the type of nimcha he is seen with.