I've seen this a number of times. There have been people I have encountered who claim that, just because an individual plays Game System X, Y, or Z, s/he "must be" a good roleplayer (in the cases I remember, it was either Amber Diceless RPG or Mage: The Ascension).
I've even seen the trend picked up in some game companies' advertising (some of Guardians of Order's ads come to mind...;).
Personally, I believe the "Oh, he plays [System X]? He must be a good player (or GM)!" to be pretty much nonsense. I have seen a number of incredible players who never played the system in question, and some prettty rotten ones who did.
Just my $0.02. :)