I saw that in RBA when it came out, and found it very helpful, and a solid classification. But since it’s bikes, there can’t be just one industry standard:
Gravel Gradient goes to 5: https://cyclingtips.com/2019/08/grading-gravel-how-do-you-define-off-road-surfaces/
What I liked about this one was the adjustment factor for “corrugated” surfaces. Otherwise, it seems to agree with yours.