He is correct. Most inexpensive A/F gauges are good for a little light show and not much else. They use stock O2's. Those are indeed narrowband sensors. Therefore, their parameters are not good enough for boosted cars.

The EGT in turn uses a probe directly into the exhaust stream. If mounted correctly, it can tell you faster when the engine starts to go lean (higher temps). The only problem here is that some cars run higher temps than others. If you don't know your car really well, you may not be able to know the difference. For example, on my car 1000+ deg temps are normal w/o any boost at all. In other cars that's lean.

The only true A/F gauge that is accurate is a wideband. That is the one that needs a wideband controller to read it. It also requires a different O2 sensor too.