Incorrect both for ich and for the cold. The cold is an infection by a biohazard, normal people don't just carry it all the time, they aren't triggered, they are infected. You can go sleep outside in the cold in wet clothes and almost freeze to death, and you'll get many things, but not the cold. You'll get the cold from being close to infected people, from being locked inside with other people for longer in the winter than in the summer, with less air exchange. There the chance is higher that you come in contact with an infected person. Fish get ich from being close to infected fish and the ich life cycle, from locked inside their small tank compared to being in the ocean, with less water exchange.
The cold really can die. And so can ich. It's genetic material that can be exterminated, not magic.
The people you saw do the 8 week method, did they do that for every fish, every coral, every snail, every crab, every rock, every piece of plastic, etc. ?