Broken O2 sensor - great fuel economy! Why?

Just got back my 2011 Gen 3 Prius 186K mi after a used JDM engine swap due to blown head gasket. Check engine light - P2238 on scanner: upstream O2 sensor heater shorted ( or engine installer crushed the wire). Waiting for OEM replacement but getting 51-55 mpg average in spite of lack of closed loop control via the O2/AFM heater. Seems O2 level is being sensed since voltage (current) is shown on my scanner. How can this be if the car is running in open loop mode?