I would guess the intent was to imply it's due to distracted driving (texting while driving or being on call and getting sufficiently distracted to cause an accident). Though laying it on the doorstep of iPhone specifically is not fair, nothing in iPhone makes it more prone to this than any other smartphone or even dumbphone model.

I also don't know whether the statistics about how many fatal accidents were caused by distracted driving exists (it's kinda hard to ask a person who died in a car accident whether they were texting before or not...) or is reliable. But given the ubiquity of both driving and cellphones, I wouldn't be surprised if there existed some data pointing to the way the parent post suggested. I wouldn't also make any far-reaching conclusions from it as the issues involved are of quite different nature, and the data, as I pointed, is probably very dirty anyway.