This is a common trope, that “Whiteness” is a construct, that there is no such thing as a White person. However, “White privilege” is defined as all the ways society gives preferential treatment to… White people, who do not exist. It’s just more semantic sorcery meant to denigrate White people. If you’re a fair-skinned person of European ancestry, you don’t (or shouldn’t) exist.
Proponents of this idea never claim that Black people do not exist. The various pro-Black political and cultural movements (#BlackLivesMatter, DEI committees, Critical Race Theory, the Black National Anthem, and so on) depend on the existence of Black people, and to an extent, on their self-identification as Black people. Since “White people do not exist,” neither can any political or cultural movements that advance their interests.