>subtyping in a more general way

This is correct. I read her paper closely. One example I give is how SICP provides two implementations for complex numbers[1], the rectangular for and polar form.

    (make-rectangular (real-part z) (imag-part z))

and

    (make-polar (magnitude z) (angle z))

then on page 138 provides this interface that both satisfy

    (define (real-part obj) (operate 'real-part obj))
        (define (imag-part obj) (operate 'imag-part obj))
        (define (magnitude obj) (operate 'magnitude obj))
        (define (angle obj) (operate 'angle obj))

1 https://mitp-content-server.mit.edu/books/content/sectbyfn/b...