It shouldn't really surprise anyone familiar with Kenneth Branagh's film work; he's a devotee of casting anyone, regardless of race/period considerations. His versions of Hamlet (set in 19th century Denmark) and Much Ado About Nothing (set in 18th century Italy) have all kinds of minorities, in the latter case with Denzel Washington as a nobleman.
On the other hand, that would've really messed up Othello, where casting Iago as (well, firstly someone other than himself) someone also
descended from sub-Saharan Africans (not technically correct in Othello's case, iirc, but probably more relevant) would have made the film completely dissonant.
It's not to the same level as that example, but it's an odd choice here too. Would you cast a white guy as Kokou, a Yoruba warrior god?