I guess this is just a looser interpretation of “airtight” than what I’ve previously experienced in life. In my experience, “airtight” means “zero exchange of air in or out”, but this setup is a controlled process where the only place air exchange occurs is through the heat exchanger. However, from these descriptions, it’s not the same sort of “airtight” as what I’m accustomed to. Not trying to say that anyone is incorrect, just that it’s different from what’s familiar to me and that’s why I was confused.