Yes it is.
> "White" as an idea to describe a group of (roughly) light skinned European people, came about in order to justify enslaving and subjugating other groups.
That's where the entire idea of distinct human “races" came about, and “white” was a pretty constant part of such taxonomies.
> Before, say, about 400 years ago, whiteness was not an idea used to identify a "race" of people.
Human “race" as a coherent, formalized idea is less than 400 years old.
Note that I'm not disagreeing at all with your idea of the role of whiteness, only with the idea that this somehow divorces it from, rather than grounds it firmly in, the idea of human “races”.
All modern concepts of race are artifacts of attempts to justify racial, and specifically almost entirely white, supremacy. The difference between them is that that basis has led to them becoming also groups of shared experience: largely, except for the white group, this is about the shared experience of being subjected to white supremacy, but for the white group it is the shared experience of benefitting from it.