Actually, that's an untrue statement. MacBook Pro's, like all of Apple's laptops, were designed to run with the lid closed (Clamshell Mode). The keyboard area is not used for venting, and the vents along the back hinge work just as well, if not better, when the lid is closed.
The only potentially damaging situation is the computer running with the vents blocked. This usually occurs either if the computer is woken from sleep while inside of a tight-fitting bag or sleeve, or used (lid open or closed) on a soft surface that fits over the vents, such as a comforter on a bed. Even then, if the computer reaches the maximum operating temperature for the CPU or GPU, it will automatically cut power to prevent damage. It's rare for any problems to develop even in these extreme circumstances, beyond the expected loss of unsaved documents.
jW