"T-thank you," Matt was surprised by Ken's display of... friendliness? Honor? Not only had he valiantly saved Matt, but he yelled. This was unusual for Ken. Matt had only known him for a couple of days and already knew he wasn't a person who raised their voice much. He would thank him later, now was the time for action.
When Ken went into Resonance Weapon mode, Matt was actually pretty bewildered and surprised, since he didn't expect that sword. He felt as though he was holding a giant crab claw, and for once, he felt short. He gripped the hilt tightly, hoping he won't drop it and swung at Sanji, yelling rather viciously for someone whom had just been infected with venom, then saved.