It’s not uncommon for triathlon athletes to not be very good swimmers.
It’s a sport that many get into later in life, not everyone has equal access to pools, and swimming in open water is much harder and scarier than swimming in a pool.
And those rocks look sharp. I would not want to be dashed against them.
I have done a (sprint distance) triathlon in conditions that were similar to this. I don’t know what happened in this race but in that one something like eight people had to be rescued from the water. I was also surprised that the swimming leg wasn’t canceled in that race.
However, I didn’t personally have any trouble swimming in those conditions, as someone who swam competitively all through middle and high school and can swim 3+ miles in the pool without trouble.
I think what makes swimming in triathlons so dangerous is that there is a wide range of swimming skill levels, and conditions that are reasonable for advanced swimmers can be deadly for newer ones. But organizers don’t want to cancel an expensive event that people have traveled internationally to compete in over weather conditions that are reasonable for prepared athletes.
I have also done a half Ironman and I think it is at that weird middle distance where people sign up for it with big plans to train aggressively, fall behind on their training, and then attempt it anyway and end up not finishing. Unfortunately, with the swimming component, under-preparation can be deadly.