Don't forget rig geometry. The angles of the shrouds to the mast affect compression loads. The height of the attach point on the mast and distance from the base of the mast to the attach points on the hulls fore/aft and port/starboard should be the same.

The stiffness of the mast will have an impact. As the mast "bows" out, the loads are no longer pure compression.

I don't know much about the designers of the 18HT. They probably designed it to some load case with a 50-100% margin and then tested it to the design case. The problem is I don't know what that case is. Some boats are designed to stouter than others. I remember a picture of a TheMightyHobie18 from testing. There were reaching with ~6 people on the boat with wind blowing solid whitecaps, to see if anything would break.