Well, I don't know ...

In both cases, the "whites" considered the "black" forces as "inferior" if not negligible.
But except for that common basic error, I think those situations were quite different :

* At Rourke's Drift, a small british detachment was surrounded by a constituted Zulu army; both were well trained in their respective military ways. Equipment was quite different, the Zulu having very few firearms... their usual tactics, their sheer number and their courage were not enough to totally crush the (improvised) strong point, manned by disciplined and non-less courageous british soldiers.
Those soldiers knew their comrades had been defeated in previous days battles (their leaders underestimated the Zulu forces and tactical abilities). And they had no other choice than to make a stand. They had no time to retreat.

* At Mogadishu, the more or less organised "rebels" rapidly saw their forces augmented by a good part of the population itself, almost totally ignorant in the "art of war" and in the correct use of the few weapons they grabbed.
This was not a "rebel" prepared/planned action against the US soldiers.
The way the Rangers armed themselves for this particular mission shows clearly they completely underrate the possibility of an armed opposition. Big mistake.