I can bring my personal experience: riding strapless, 23-25knots, board flown on my head. Without helmet I would have probably got unconscious. It takes seconds to drown.
Every incident has its own story, causes, etc. but saying that helmet never helps it's quite misleading, in my opinion. How can a protective gear do not protect you at all? will you allow your beloved ones have kite lesson without protection gear? Don't think so.
I would love to read any kind of proper study/statistic on high speed impact in water sports if there is any.
Don't think that "there are no flying rocks from underneath a truck in front of you, you won’t hit asphalt when you fall" is the point. If you fall from high enough or fast enough, water (or board) can be pretty hard.
Last but not least, you never know who's riding next to you and if they decide to land on your head.
I think it's plenty of reason to wear an helmet, also protect you from the sun, or cold, protect your ear drums...
Same thing for the impact vest, will it safe your life? not necessarily, but even if it's the 0.1% of chance it does, will you skip it?
And it add a bit of buoyancy, not a lot, like a proper life jacket, but better than nothing, isn't it?
Regarding this topic, do I understood correctly that the rider was megalooping in shallow water?