It's always bizarre where they draw the lines for Whiteness - this statement techinically only covers English-speaking Brits, and Germanic peoples... while other people, historically, didn't count Germans as White, but as 'Ethnic' (which is literally everyone on the goddamn planet.)

Whiteness has no consistency except to exclude as many people as those in power wish to exclude, and to act as a carrot to those on the fringes - Celts, Italians, Greeks, non-PoC Jewish people, etc. - to impose the same kinds of oppression that they face onto PoC in turn. It's creates a horrible hierarchy which keeps those at the top happy while everyone else is too busy fighting each other to notice the dudes at the top getting rich on it all. Sadly, a lot of descendants of the Celtic nations are willing to betray their heritage by using it as symbolic of White supremacy (my heart breaks every time I see the corrupted White supremacist version of the Celtic cross.)

...Sorry, got a little off track.

TL;DR: Whitness is bad, arbitrary, and inconsistent.