Yep, strength, but they need some weight aswell, Peteys bodyslam comes to mind and now Luke Hughes who got tossed around...these lightweights can get hurt pretty easily. Upper body muscles are great for protection, padding if you will, but as you say a powerful lowerbody and core are most important.
I´m sure they have good strength coaches, and it takes time to put on muscles so let´s see where they´re at at 25.