Some years ago the 3 year old son of a family friend drowned. There were latches above child level on the pool access doors. While his mom was napping, the child awoke climbed out of bed, pulled a chair up to the door, unlocked the door and got out to the pool area. He was found no more than 10 minutes later.
She had no reason to think he was old enough to figure out how the latch worked and how to move a chair so he could reach it. Accidents happen.
In the case you give, it does appear that the parents took reasonable precautions. As I noted above, most of these cases involve 3 to 5 year olds.
Now, if you told me that a one year old had done that, I probably wouldn’t believe the story.
But your child can always do one more thing than you think they can. That’s why you can’t rely on passive measures.