Initialisms apparently naturally have a hierarchy, like when lesbians invented the LGBT and graciously donated the other letters to good causes.

I suppose it would be more similar if it went from "Queer" to "LGPWAQ", as in "Lesbians, gays, and people who are queer".

BIPOC definitely reads like "Black and Indigenous people, and the rest." Which makes sense since the intent of the term is to "center the experiences of Black and Indigenous groups." Can't effectively center something without moving everything else out to the margins.

The ranking is the point of the initialism. Black and indigenous people were already included in the blanket term "people of color," so why introduce BIPOC if not to highlight those groups specifically?

Exactly--and the Sierra Club guide discloses that exact motivation in the part I quoted above.