You might be closer to reality by saying "most"... likely just "many", but certainly not "all" Wave races are IWCA. Maybe... "all IWCA races are under IWCA rules".

The Hobie Class Association does offer events for Waves which are under IHCA rules requiring a Hobie Class approved sail. I have personally sailed in HCA Wave events over here on the West Coast, a number of times. Fyi... the HCA events that I have sailed have been focused on entry level and youth. I sailed those events with my son.

Note that the standard and class-approved sails from Hobie Cat are sometimes the less expensive option as well. Some of the custom sails being pushed are not cheap.