Duster 340 not a muscle car...

I guess it comes down to the interpretation of the person that made the statement. If he meant to 'slam' 340 Dusters when he said it, then he was implying that the 340 Duster wasn't much in the way of performance. If he was going off of some of the definitions stated in this thread he may not have been besmirching the Duster's reputation.

These days most people I know use the term 'musclecar' when they're talking about any car from the mid-60s through the mid-70s that had a high horsepower to weight ratio. - Regardless of any official definitions used in the past.

Most likely a Chevy boy making that statement meant to imply the 340 Duster didn't rank in horsepower or prestige with Camaros or Chevelles.