Probably me not being clear, but we agree and I was trying to say what you are saying, but in a less clear way.
By saying a shift on the curve I meant by increasing ppO2 we shift how saturated our haemoglobin is at the lungs, which corresponds on the graph to a shift along the x axis and up the y axis. The amount that shift occurs is (as you say) described by the Hgb-O2 dissassociation curve. Hence, an increase of ppO2 at the lungs (e.g. by shifting from elevation to sea level, or by increasing %O2 in inspired air) can be visualised as a shift along the x axis, or a shift up the curve (from the point of view of the blood oxygen saturation returning to the heart from the lungs). I realise that wasn’t very clear when I first wrote it! Not sure this really clarifies, but we do agree 