This is a very insufficient article. Almost useless. It doesn't mention anything about hip and sacral alignment, which is absolutely critical to having correct posture. Unfortunately, sitting in a chair, as well as various other modern activities, completely messes up not only the alignment of the hip and sacrum, but conditions the soft tissue in those areas to hold a bad position. That's why it's so hard to have good posture for modern people - our bodies are molded into the wrong shape and are holding it.
Suppose an arch's keystone is pushed out of the place? The arch collapses, and can't support anything on top of it. The sacrum is that keystone.
For real progress towards fixing posture, have a look at the explanation presented by the Olympic athlete in this video.