How is that chitin resisting the 4000 Newtons, in terms of the orientation of the forces being exerted on the material? Tensile strength? Compressive? Resistance to buckling? Stiffness?

I sincerely doubt that value is a fair or meaningful comparison to a human spine any more than, for example, me letting everyone know that chicken eggs have an ultimate strength of 14,000 Newtons if you apply the load properly.

I'm perfectly willing to believe that the rest of what you say is true, but that specific bit really stuck out and cast doubt on all what you said, for me. A solid block of calcium carbonate would be impossible to crush with your hands, but, well, eggs are like eggs are, because of how thin that incredibly hard shell is.

If those paleoarthropods had thicker exoskeletons relative to their other proportions, or if we had some numbers backing up that chitin with less protein in its matrix has a higher tensile/compressive strength, I think maybe some of these 2.5m creatures could resist a guy with reasonably good boots on, but I don't think you've shown any of that to be true yet.