It might be just a quirk in the TuneEdit software. Before putting in a DJ O2 sensor bypass: 1) E-mail Wayne and ask him if his program is really turning off the sensor when you download the program; 2) Assuming that your new L tables have reduced the backfiring, you should notice increasing backfiring over time as the ECU adapts if TuneEdit is not turning off the O2 sensor.
Actually, it's not really turning off the sensor. It's fooling the the ECU into thinking that conditions for closed loop (low throttle position) are never met, so it never uses data from the sensor - i.e., it always uses the data in the L tables for low TP. This is how the DynoJet O2 bypass works.