One of the nice things about a metal chassis is that it conducts heat very well. Now obviously, the screen isn’t metal so closing the lid probably does trap some heat but the metal chassis itself does not trap heat, it absorbs it and then radiates it to the outside world.

This can indeed give the impression that heat generation is excessive in a metal chassis laptop, or that the thermals suck as you say, because the case can get warm or even hot very quickly. But that is actually because the chassis allows the heat to escape.

On the other hand, a plastic chassis will stay much cooler on the outside because it is very effectively trapping the heat and only forcing airflow with a fan allows the computer to stay at an appropriate internal temperature.

In other words; if all other parameters are the same, a laptop inside a metal chassis will have a lower internal temperature and a higher external temp, while one in a plastic case will have a higher internal temperature but the outside of the case will be cooler.