Yes, this is certainly true about pets.
It applies equally to children. So far this year
19 children have died from heat stroke inside a vehicle. In 20 minutes the temperature inside a car can increas by 29 degrees. Cracking the windows does not have any significant effect, possible no effect.
If I see a child unattended in a car, I call 911 first thing to report it. In California, especially in our 100 degree plus current temperatures, police take this seriously. That five minute trip to drop off some film or buy just a Coke and a bag of chips can easily become 15 minutes without your thinking about it, and in that time that care that started out at 80 degrees is now over a hundred.
Never leave a child unattended in a car for any length of time for any reason.
All stats in my post come from the referenced web site.