In my previous playing life, I would treat the black notes as super quick grace notes. Since those are open strings, they’ll continue to ring, but the notation is just differentiating double vs quadruple stops.
Very common notation and I’m curious to know how to do it in Dorico as well.
As a cellist, I agree with this interpretation. Multi-stop chords must be rolled one way or another (often from the lower strings to the higher strings), so I think this is the composer’s way of saying treat the bottom notes as a grace and get the more important notes (on top) quickly and on beat.
But even without this notation that’s how I would interpret it too haha. I guess the idea is to ensure there’s no slow rolling chord.