Let’s argue about this one weird chord in the Brahms Intermezzo in B-flat minor

I have some aspirations in music theory pedagogy, and toward that end, I’m learning more about Schenkerian analysis. If I’m going to resist it, I should at least be conversant in the thing I’m resisting, right? So I’ve been reading Schenkerian Analysis: Perspectives on Phrase Rhythm, Motive and Form by David Beach. One of his …