Umm... no. Your legs are not part of your core when spoken of by fitness experts. Midesection, back, chest, and glutes are the usual definition. So you are still wrong. I've got 23 years of combat and fitness training that says so.
Erm, you guys are both off.
The "core" contains your tranverse abdominis, rectus abdominis, obliques, and spine erectors.
Strength (arm and or leg) has little to nothing to do with KO power. It's all about technique baby.
It's like throwing a ball. Some people throw a MEAN fastball, some can't.
If you watch GSP on Criss Angels show (Youtube 'GSP Criss Angel') they both hit a punching machine and Angel kicks his butt.
I'm sure GSP can hit hard but he doesn't have great natural KO power. ANY guy in the UFC could KO someone if they catch you standing still.
This is just my opinion of course...