The sensor is likely a simple wind vane. Since there are no leading edge devices, stall AOA is likely very close to the same value, relative to the leading edge, whether the flaps are up or down or in between.
In Navy airplanes I flew that had leading edge devices, there were markings on the AOA gauge to discriminate between clean and dirty stall AOA. Still, a simple (or sometimes a bit more sophisticated) wind vane was the basic sensor.