I have plenty of experience with engine performance, and s/he was absolutely correct. An overly loud engine is is opening its valves too early or closing too late, and it can only be made quieter by restrictive mufflers. More modern and efficient engine designs, enabled by technologies such as variable valve timing, are quieter out of the valves because they convert more energy into motion instead of sound. Being quieter, they also have less pressure, and are less affected by muffler restrictiveness. Straight pipes might make a 1969 charger more powerful, but the effect on a modern V8 like the BMW N63 is negligible.

The most dramatic example is VTEC, especially before variable cam phasing.

Every joule which goes into making noise is a joule which isn't being used to make the machinery go.

The energy lost to sound is minimal, and often there simply isn't an economic way to prevent those losses without reducing performance.

Tuned exhaust are only "in tune" at a particular rev range where the resonant frequency of the exhaust manifold is in sync with the engine combustion cycles. They're not widely used outside of racing applications, because a road vehicle has relatively wider rev range than a racing machine. And they're ridiculously loud.

A vintage Cosworth DFV racing engine only "works" at 7000-9000 rpm (or 8000-10000 rpm for later models), a pre-turbo 2000s F1 car runs at 11000-15000 rpm to about 14000-20000 rpm (before rev limits were enforced). It's a really narrow band compared to a road car that can run from 1000-6000 rpm, ie. is useful on the entire rev range.

All modern engines have well engineered exhaust manifolds, though, but they're not really "resonance charged" like a tuned racing exhaust.