yes its an anecdote but logically I don't see the structural curvature of the vertebrae you were born with changing... and that's what it is, unless proven otherwise...
Changing something you were born with is completely different (and in this case i'd agree that you're probably right) from changing something that has been a result of adaptation. Logically, this makes sense. However, what isn't logical is taking something that has changed due to various repetitive demands over a certain amount of time, and being unable to change it back, with various other/opposing repetitive demands over time. If it got there in the first place, then, logically, you can get it back. Most of us were able to do perfect squats and deadlifts when we were toddlers but can't after years of inactivity. We're not stuck with this, it can be reversed.
Anecdotally, I see improvement to at least some extent with every single person I train (i have also quite dramatically improved my own resting posture over the years). I've also had clients tell me they have noticed improvements in posture without me even specifically mentioning static posture to them (i rarely mention static posture with clients, btw) i.e. "i've noticed I stand up much straighter now". Also note that I don't view "posture correction" as a goal in itself. I view it as an indication of weakness and limitation and use this information to help get someone towards their goals. For example, someone with kyphosis will almost always have poor horizontal and vertical pulling strength. Most people I train get "too many" rows but also with the emphasis on getting considerably stronger at the movement and not just doing a bunch of light reps hoping something magical will happen.
Have you ever seen someone with an impressive "middle" back who has a hunched posture? Or someone who can do rows and pull ups with a tonne of weight (and decent form) with a hunched posture?
Similarly, ever seen a bodybuilder who DOESN'T have internally rotated shoulders? (then consider the emphasis on chest and shoulder training in bodybuilders).
My point is - what actually defines or contributes to someones "resting" posture?
In other words - what got you there in the first place? Can it be reversed? (generally something "structural" can't be reversed, btw).
there's plenty of people who are slouched over desks and computers all the time yet have "normal" lumbar and thoracic curves so you can't just put it all on tight this or loose that.
If this is the case, I rarely see them. On those that do have a decent posture, they're almost always quite active outside of work in one way or another (lifting, sport, hobbies).
"tight this or loose that" - I actually agree 100%! You should "think movements, not muscles".
To get more specific, what I think is lacking in most of the information available on correcting posture is the emphasis on strength i.e. it's one thing increasing your push:pull ratio in favour of pulling but are you actually getting stronger in the pulls? If not I don't see it working. This is actually mentioned by the likes of Cressey as is the emphasis on the other 23 hours of the day outside of the gym.