I doubt they ‘rappelled’ on the knotted sheets.
I have used carabineer brakes, figure ‘8’s, hot seats and more but never on a knotted rope!
A hot seat rap with knots on vertical would very time consuming.
Most likely they climbed knot to knot.
Never having used bead sheets , I wonder how much stretch? Like an old length of Goldline.
Just call them biner’s or crabs.