He's one of three people in the world who can see and he has far more knowledge to go along with it, yet he lets a few blind guys sneak up on him and take him hostage? Guess all those books didn't imbue him with common sense.
This was Kofun and his sister's first time seeing the outside world. They were in shock and awe at what it really is.
They witnessed a group of seemingly innocent men get burned alive. Kofun did puke, and he looked like he was trying to mentally recover from all he's seen. At this point, he wasn't paying attention. These men are slavers and they seem to be good at it, so it would reason that they would know how to sneak up on someone, and flank him without making a noise.
On top of that, the 2 kids are inexperienced, being in such a hostile environment for the first time (against other humans).
I don't doubt they know how to move silently, but they would be 100% ignorant of sightlines. Also the silence and blindness would make for an extremely slow moving individual compared to a person that can see. Kofun was just a useless waste of sight.
Kofun wasn't paying attention. He probably had some form of PTSD where the body and mind are just out of it. When the people go through trauma, they don't just "snap out of it" when they need to. He wouldn't even think to be scanning the areas around him for threats. I also don't think anybody ever warned him there would be slavers on the loose.