Posture gets blamed for almost every sore back, and the evidence for it is weaker than you would expect. What usually matters is how much your back has been asked to do recently, and how little it has been asked to do before that.
The desk setup advice is not wrong so much as it is aimed at the wrong variable. Sitting upright does not protect you. Sitting in one position for six hours without moving, then standing up suddenly, is closer to the actual problem.
Backs are not fragile. They are adaptable, and they adapt to whatever you ask of them — which cuts both ways. If your week has been almost entirely still and then you spend Sunday lifting boxes, you have handed your back a demand it has had no practice meeting. That is when it complains.
The useful question is not "am I sitting correctly." It is "what is my back doing across the week, and is the amount trending up or staying flat." A short walk after lunch, a few minutes on the floor, and lifting that starts light all do more than an ergonomic chair ever will.
When pain has already settled in, we usually start by calming it down, then rebuild the amount your back can handle in small, deliberate steps. It takes weeks rather than days, and it holds, because you have changed the load instead of changed the chair.
