It's been pretty robust in my experience. On some of the scratched stanchions I've seen it's like it's slightly chipped along the scratch. I haven't ever seen it continue beyond that afterward, and I've seen a lot of riders run with scratched stanchions, Kashima or not, without further issue. It's good to remove any raised edges.
A thick build of hard anodizing will also chip similar to Kashima. IDK the build thickness used by FOX on their black stanchions. We used a lot of various thicknesses of hard anodizing at work, up to 0.003" IIRC. It's very tough and scratch resistant as the hard coating is essentially sapphire.