But the author used a wrong chord description. This is a polychord (one chord on top of the other), but the chord description is for a chord on a bass note.
Just to avoid misunderstandings.
The correct one would be
B
—
C
Well spotted - he’s written a slash chord (diagonal line) where he seems to intend to write what I was trained to call a polychord fraction (horizontal line). Still cool stuff though