Firstly, the SPLC is a serial defamer and holds truth as a lower priority than essentially anything else. As a matter of principle, I won't defer to them on anything at this point.

> "Western chauvinist" literally means "white supremacist"

Only if you make several massive leaps in how words "are used". Has it occurred to you that the word "chauvinist" was chosen specifically to get a rise out of people who are overly sensitive to language?

If it's really about racial superiority, how do they countenance being led by a black man and composed of a great number of non-white people?

Then, if it's really about "white culture" (and I'm not really convinced it's thought of this way inside PB), then how is that white supremacy? Anyone can adopt a culture, and there are lots of "white" peoples in the world with objectively excellent and productive cultures worth being proud of; liking England or Norway doesn't make you a white supremacist. Enrique Tarrio, the leader of the Proud Boys, spells out explicitly that there are nations outside the West who have adopted the elements of culture that the Proud Boys are "chauvinist" about.