I don’t know anything about the existence or magnitude of the ground effect in question but I may still be able to help.
Once upon a time a ground effect device fitted to a Harrier increased the VTO weight by 1500 lb. Its effect tapered rapidly with height and had vanished by about 10ft wheel clearance.
When doing a normal VL the same device produced a negligible reduction in descent rate. The question is why?
Mr Newton helps us to appreciate what sort of acceleration or deceleration we get from applying a force to a mass. Any ground effect deceleration of the chopper during an autorotation to land (even if the deceleration was say a huge 1 g) would only actually reduce the touchdown rate if it was applied for a length of time. And time is likely to be very short doing a VL through ground effect.
Bottom line is any positive ground effect needs time, and quite a lot of it, to reduce a sink rate. And time is very short in the manoeuvre under consideration.