The fact Rittenhouse was fleeing in the first place should say that he did exhaust all other reasonable means of escaping death or bodily harm. The fact that Rosenbaum, Grosskreutz, and Huber all chased him down, one with a firearm, all three of them had the intention of harming or killing him, should tell you that all of his safest and most reasonable options had been exhausted. Rittenhouse was knocked to the ground and in a vulnerable position. The armed assailant was approaching point-blank range.
(In fact, you could say Rittenhouse was exhausting his last viable option when he was attacked--which was escape).
Reasonable self-defense.