Just my two penneth worth. I think it’s deffo Kate but her facial expressions and her walk are on perform mode, walking like she knows she’s being filmed. Smiling and slightly self conscious. Normal people walking to the car would have a resting face on (in my case total resting witch face)
Have you even seen the video. They come in frame and Kate and William are both talking/laughing with each other. That's what you see. A couple walking/talking/laughing.