Yes, looks like a light for refueling, and the location would probably be chosen to minimize blinding of the flight crew. Alternatively it could be a tail camera as seen in large airliners (A340-600, 777-300ER, A380, A350-1000) to make ground maneuvering easier in tight spaces, but it does look more like a light to me.
I guess now is beyond doubt that WS20 is entering final testing before start production , a big advance for Chinese Engine. Now that all 4 engines are powering the Y20 instead of 1 for the earlier test. Does bigger radius mean bigger fan mean higher bypass ration translate into more fuel efficiency?
Stubbier design mean fewer compressor and turbine stages?