There are a lot of factors that cause this
That I agree with. Heat stroke tends to accelerate other underlying conditions that a person may have. Not drinking enough water also accelerates other underlying conditions that a person may have.
Antidotally, you believe that he died from heat stroke. That is fine. The heat stroke may have accelerated the actual cause of why he died. Without an autopsy to confirm the cause of death we are both facing into the breeze and getting wet to boot.