As a white woman, the important (& ultimately freeing) thing that our sister said is that whiteness is not an ethnicity.
IOW, white is the category that US history, culture, & society has used to classify me & separate me from POC.
On the other hand to persist in this category, to protect it, is to embrace an inherently flawed, hierarchical system.
Even tho we don't realize it, many white Americans think of ourselves thru the racial lens of whiteness, confusing it for our ethnicity. When we do search out our ethnic DNA, it's is a hobby, a curiosity. We want to know "where" we're from.