You talk about it being public knowledge that there is an ‘asterisk’ by ‘we the people’, as if slavery was some secret.

Also how does Dred Scott support this innuendo?

There was nothing secret about the ideals, and nothing secret about slavery, and nobody was surprised to find that slaves weren’t considered to be citizens. This discrepancy was a source of some public disagreements.

The retconning is the idea that the ideals were somehow a fraud, rather than an extraordinary step forward at a time that was far more brutal than today, and that people had to work hard to try to establish the ideals as more than just words on paper.