He did understand.

"I'm calling it the brain! I'm pretty sure it's where ideas come from!"

He understood because he was the one that had invented the word for 'brain' in his language, because he'd figured out that the different parts of animals had to each do something and hitting them on the head when hunting them made them go all wobbly. Getting hit on the head himself made it hard to think. The brain was important and all animals had that squishy stuff in their heads and had hearts like people did so he assumed that people had that same squishy stuff, too.

So he could understand that, without any knowledge of biology or medicine.

"It hurt your brain, didn't it." He paused. "Um. I don't know if that word translated to one you have because I'm the one that made it up. The squishy thing." He tapped his head. "Where ideas come from. Did that get hurt so those ideas that made the parts of you got stuck as separate people?"

Maybe that was why you saw double sometimes when you took a hard hit to the head. Maybe that was the parts of you almost splitting into two people.