Unknown Centurio Graz museum

[Renatus]

It doesn't matter what some 18th century soldier says, cuirboulli is nowhere near as hard as steel.

I don't suppose that anyone seriously believes that it is and I don't suppose that the 18th century soldier carried out a scientifically evaluated comparison test to ensure that his statement as to its hardness was 100% factually accurate. Nevertheless, he and his comrades would appear to have believed that it was hard and offered some protection and that it was certainly better than nothing. I would suspect that most impacts were likely to be glancing blows or, at any rate, not 90 degree direct hits, so the sort of impact forces that you cite are, perhaps, not fair tests of the general utility of cuir bouilli.