There is no such thing as "dry rot" anymore. That is a holdover term for what would happen to tires that used cotton plies many years ago. Now days it's called weather or ozone checking, otherwise known as oxidation from the sun and extreme temperatures.
As far as pic. The tire is checked. However it doesn't appear that's the cause of the sidewall failure. That appears to be an impact pinch. On a tire in that condition it would not take a very hard shot to cause this damage.
Take my word for it. I been in tire industry a long time and have certificates up to instructor.