Yeah I mostly agree with you. I give the parents some grace, as rulebooks are complicated, and they might not have even known which one they need to read (and as a schooling show, it didn’t even have any governing bodies mentioned on the premium that might have pointed them in the right direction). Most parents leave that sort of stuff up to coaches anyways, especially if they’re not “horse people”.
But I definitely think the dressage coach should have known better - even if she didn’t know H/J rules specifically, she should have known to look it up (free and online, as you mentioned!).