Yes, probably. With the evidence presented, it probably wouldn't have been self-defense but I don't think this evidence would have been available in that case. Grosskreutz appears to have chased down Rittenhouse because he was a "bookaloo boy", which means he instigated the fight. But again, I doubt that evidence would have been presented and definitely would not have been confirmed by Grosskreutz in testimony, if he was on trial.
Defense for Grosskreutz is "I heard a shot, saw a guy running with a gun, the crowd said he's the guy! He shot a guy! I ran over to stop him and he pointed his gun at me, so I shot him."
IMHO - Both have a valid claim of self-defense.
Even if Rittenhouse had not shot anyone, but Grosskreutz thought he was going to, then yes because Grosskreutz can show that he reasonably feared for his life. . Even if they both shot each other and neither died, both could claim they legitimately feared for their life and shot in self-defense.