I don't think it's a myth, it's more like a misunderstanding.

Exhaust is NOT steady-state flow; if it was, then the bigger the better. You always want to allow better exhaust flow, but sometimes a smaller pipe is the way to do just that. It's not back pressure, but it's the exhaust velocity, speed-of-sound, & reflected pulses back into the exhaust ports. They don't cover that stuff in a basic fluid-mechanics course.