This is correct. Part of the science of heat dissipation is having somewhere for the heat to go. When in hand and used as the session vaporizer we build, it is engineered for session duration and continuous use. Not to be put down or left unattended.Although the heat is not good for the overall life of the vape, there is nothing in the isolated vapor path that is affected by heat. And my experience with portables used with a bong says they all get hot . The one I have that heats up the least is the Solo 2. My Tera, CFX, CFX+, and my Argo all get pretty hot when used with a bong.
Why?
First, because that's a waste of product - that oven is on CONTINUOUSLY. And also because there's nowhere for the heat to go when it is a session vaporizer.
So when used in a bong, or anything that is essentially not coming in contact with anything else (the shell), heat will just build and stay in the shell. When you have it in your hand, your body heat equalizes the heat and absorbs anything extra.
Also, to the point of the "life of vaporizer" - the same logic applies - as the more in your hand contact the vaporizer has, the less the heat builds within the shell. Your body acts as a heat sink, pulling heat out of the unit.