I understand your point but that gunk he is talking about is build up of those chemicals, I'll use your tea analogy from earlier, you put a tea bag in water you'll get tea right? It's always going to be tea right? but if you were to burn that same tea bag and then put it in the water you will end up with tea that has carcinogens in it.Any other chemicals produced (such as acetoin/formaldehyde) are produced when heat is applied. So even if you put in a brand new coil after every single puff, if you start with the same base chemicals and apply the same heat you are going to get the same result, every single time.
The chemical composition of Propylene Glycol, Propylene glycol is used with things like flavorings because it is incredibly great at suspending particles, to go beyond that, doctors/hospitals will actually use PG in conjunction with a nebulizer for drugs that are not very water soluble since PG suspends them better now with that said it is 100% likely that the black gunk, the build up is leeching into the PG and then being aerosolized and inhaled and since his coil is so old I can be 100% sure that he is inhaling more toxins than he would be if he were instead using a fresher coil since there would be less build up to suspend.
I am not saying the levels are very high or anything like that, there's no way of telling without a study being performed I'm just saying letting it go that far will reduce the ratio of harm reduction vapor products offer which will obviously still be less toxic than that of tobacco Cigarettes.