Nowhere in the article does it say that the happiest people are those with nothing.
Of course the article doesn't come right out and say that. If it did, everyone - maybe even the author - would immediately see the fallacy.
It does state that those who are less materialistic are happier but that doesn't equate to having nothing.
But it does imply that, from the very first sentence: "Our piles of crap don’t just contribute to reality-TV shows like Storage Wars and Hoarders — they also make us miserable..." It is stating that it is
having stuff that is making people unhappy, not the materialistic attitude that leads them to acquire piles of stuff they they don't use.
Those that don't place value in having things can still have things.
But why would they? Obviously, because having a certain amount of 'stuff' can improve your life. It's just finding that Goldilocks point where the stuff - say a bike - improves your live (you can go places, get exercise, etc) but doesn't cause debt-related stress.