I measured a thickness between 0.4 and 0.45 mm.
I suppose Slic3r can't print 0.5 mm because the nozzle is 0.35 mm and the layer height is 0.25 mm, indeed it prints only one line each layer.
If the thickness of the ooze-j-test would be 0.8 mm or greater than Slic3r would probably print at least two lines.
Otherwise setting the layer height to 0.2 mm results in a wall width of 0.49 mm.
Edit:
I've attached the Gcode-File with layer height=0.2mm > thickness = 0.5mm