My guess is it may be just a 1x1 twisted rib where you work in the back of each k st instead of the front just on a larger needle? Like a 8mm+ with dk or 5/6mm+ with 4ply? Good luck!
To be specific,
This photo would be what slip rib on a top down sweater would look like, turn it around for bottom up (what your swatch would look like)
I was very tired when I wrote that… That isn’t exactly how slip rib is done…
The slips only happen on one side… There are plenty of videos… Sorry if I caused any confusion