"To suggest keeping your baby in the car seat for half an hour to an hour (after you have been somewhere in the car with them) may result in their death, is nothing but histrionic scaremongering!"
It's not, actually - it's been shown by research that this happens.
It's RARE - it's not a given - and it affects very SMALL babies ie premature babies, or full term newborns - it's not going to be a problem for a bigger baby e.g. a four month old. But it's not histronic or scaremongering. There have been links posted to the original research and to the NHS breakdown of that research, which is very good.
This is the mechanism of the danger, we know exactly what happens:
Baby is placed in the car seat which is at about a 40 degree angle, for safety, in the car. Sometimes the angle itself, and sometimes the movement of the car can cause some small babies to fall into a chin-to-chest position, which makes it hard for them to breathe as it creates a bend in the airway. Try it. Place your chin onto your chest and you will find breathing is suddenly more difficult, and you need to take deeper breaths in order to get the same amount of oxygen. If you stay in this position for a long time it's uncomfortable, and you'll start to feel light headed.
Very little/new babies tend to have more difficulties breathing than adults or older children anyway, because they are new at it and their respiratory systems are much more delicate. In addition, very small babies have extremely limited head and neck control, and cannot move themselves out of this position or adjust their heads in order to breathe more easily, especially when they are still in the car seat which is at an angle. If an older child or an adult falls asleep in the chin-to-chest position, we will tend to wake up and move. A newborn baby cannot do this.
So the result of the poor breathing position results in reduced oxygen saturation levels. In itself, this is not dangerous. You can get fairly low in oxygen sats before you're in serious trouble, which is why it's not inherently dangerous to use a car seat with a 40 degree angle. Indeed, the safety benefits from crash protection outweigh worries about lowered oxygen saturation levels for short periods of time. The problem is that over time, the difficulty in breathing leads to oxygen saturation levels getting lower and lower. The normal/healthy range is 95-100%. Anything under 90% is considered a concern. Anything lower than 80% can begin to affect the workings of major organs, drop to 65% and you lose mental capacity, lower than 55% and almost everybody loses consciousness. Of course, at this point the person is in significant medical difficulty - in hospitals a drop under 90% is considered an alarm and oxygen would be given at 80%. It's not a state you want a newborn to get anywhere near. (In the recent research, the test was stopped if any infant reached 85% - which some of them did several times during the 30 minute period).
There is older research which shows infants' oxygen saturation levels steadily decline over time when in car seats - you can read the full study here: pediatrics.aappublications.org/content/108/3/647?ijkey=3fb9d84135563d7a4e5344c8488e7043cbcf06cb&keytype2=tf_ipsecsha
Most infants who are healthy and full term aren't entering the danger zone, and probably wouldn't for several hours - but some will after a relatively short time. In addition, this testing was performed in a static car seat placed on a flat, unmoving surface - IRL the car seat is being moved around, placed in cars, driven over bumps, around corners, subject to vibration of engines etc, which is likely to exacerbate the poor positioning problem, and cause saturation levels to decrease faster than this older research suggests (this is what the 2016 study was looking at).
The point is, if your baby has been in the car in their seat, they probably have less than perfect oxygen saturation levels. When you take them out of the car, but leave them in the seat, this does not give a very small baby the opportunity to change position. For SOME babies (but not all) being left in the same position for a further 30-60 minutes could be incredibly dangerous. Without an oxygen saturation meter, you wouldn't necessarily know.