I think that's a big overstatement.
Here you can hear the Friedman Plex with its (afaik PPIMV) turned down. Soundwise it just gets cleaner sounding, less "raw" sounding. Does the fully cranked setting sound better? Yeah, but the MV is still a usable option if it gets you to a palatable volume level without involving expensive attenuators.
I think more modern amp designs can also be voiced to sound like those classic amps without ever involving poweramp distortion.
When I had the Vox-based Victory VC35, I also tested the poweramp distortion by running it into a Fryette PS-2. Guess what was needed to replicate that behavior surprisingly well? A Dynacomp pedal up front to add some compression, and turning the amp's preamp gain up by a few notches! It was so close to its cranked sound/feel while still using its master volume to reduce volume.
