I think the phrase is massively misinterpreted.
Being poor does not make you miserable. It just means you will face different stresses than if you were not poor. Poor is also relative of course, but I've been very poor and not miserable.
It all depends what you value in life and what actually makes you happy. Being with my child makes me happiest.
We all also get so much joy from small treats, which when not wealthy cost under a tenner and can bring so much joy it's euphoric. To get anything close to that kind of euphoria as a wealthy person you'd have to spend thousands, and then what? You can't do that every day.
Happiness is an intrinsic thing, not something you can buy. When I'm truly happy you can throw just about anything at me and I'll get through it happily.
But poor is also very relative. Even when you're skint but have a roof over your head and an internet connection and a fiver for some crisps and wine, you can still find joy in life.
It's just not about money, it's about mindset.
If you're talking about living in an impoverished country it's completely different. Firstly say you're poor, well you're going to be something quite a bit worse than miserable. Say you're wealthy, great now you're surrounded by horrific poverty sipping champaign and if that brings you intrinsic happiness you’re a bit of a sick individual.