You're right, you don't understand it :)
But I can help.
Thomas is just referring to a longstanding part of constitutional interpretation that says to look at different sources including text, intention, and tradition, with tradition being one, but only one, of the factors.
Thomas was saying that the precedent was never valid since it wasn't founded on preexisting tradition. The precedent was therefore always arbitrary, a previous court doing whatever the fuck they want.
To be clear: this isn't some post-hoc rationalization. It's a longstanding and common approach to the task.