Two more factors are important here: the tautness of the canvas, and—especially—the angle of incidence. You can see how Ross is holding his knife at maybe a 20° angle here:
I'm mostly mentioning this because you make no remarks about these parameters, and for all I know that's not where your problems stem from. However, seeing the photos of the marks your knife leaves on the canvas, it looks like that could be the issue.
Have you tried using the convex side of the palette knife?
Also note that Ross has quite a lot of paint on his knife, and that he doesn't scratch with it: he seems to use it like someone delicately putting a lot of butter on a sandwich.
I myself have only used smaller, or, at least, narrower, palette knives. A short, rhombus-shaped one, which was very rigid, and good to use for making small scratches in the paint, or even just plucking hairs or other unwanted particles from the surface; and a very flexible, 7 cm long narrow one, which, while not being able to carry as much paint as the one you and Ross are using, would allow for similar applications.