This is interesting!
I’ve done ewaste gold recovery myself. Typically you need to dissolve away all other metals, wash, then soak what remains (gold bits and bits of fiberglass) in aqua regia which dissolves the gold but not the fiberglass.
Then filter out the fiberglass and precipitate out the gold with sulfur dioxide.
The end result is nearly pure gold which is better than the 90% in this study, but purifying gold isn’t that hard, and this saves time in the steps where the circuit boards are involved.
Also I suspect this lets you re-use the aqua regia. I think Nitric Acid production involves some greenhouse emissions.