He was getting hit to the body way before the KO. It was accumulated punch to the body not just that particular shot.
^ this. - and quite obviously. Less than ten seconds before the KO, Rojas threw a left hook that would likely have KO's Arce if it had landed. Also, after he went down, you can see him immediately try to get up again, but he isn't able to due to the pain, or a lack of breath. Body shots can do that to you.
If anyone has ever had a "glass body" it's Rojas. I thought Rojas was winning that fight easily, but then again I also thought he was beating Darchinyan who knocked him out on a body shot as well. I can't say I buy it too much. The only thing that gives me doubt is that Nashiro is also a big body puncher, and he managed to stay on his feet for that one.