Yes, I should have stated a planar wing. XFLR5 routinely indicates span efficiencies considerably over 1 for a planar wing. It cannot properly even sort the relative efficiency of a rectangular wing with square tips, an elliptical wing with a straight 1/4c, and a crescent shaped wing, compared to good wind tunnel testing.
Yes, non-planar wakes are interesting, but VL or LL code cannot not model that. You need pretty radical geometries, like the split tip wing proposed by Smith to perhaps have span inefficiencies over 1 for a planar wing. XFLR5 shows span efficiencies over 1 for a rectangular wing, which is not correct.