I read Rosemary R. Reuther, and decided that I agreed with her that the myth of original matriarchy is a total invention based on crap scholarship by a hack who seemed to think that inventing a glorious history for womankind would give womankind a glorious future.

Actually, she's wrong about that. Cultural evolution, including the idea that matriarchy was the oldest form of kinship and social organization, was accepted anthropological theory when modern Wicca was getting started (and Gardner was receiving his education). It was later rejected, but that doesn't mean it wasn't widely accepted by scientists at one time. Wiccans definitely held on to the idea long after it lost currency elsewhere (one of the dangers of working science into your religion, actually), but they did not just make it up out of whole cloth or adopt it purely because of a feminist agenda.