Statistics say otherwise. KG, outside of 03-04, never had a DWS season over 7 (he had a phenomenal 8 that one season, his next highest was 6). Duncan on the other hand, had 3 (with another two seasons of 6.9, and one with 6.8).
In terms of overall WS, Garnet had one phenomenal season (again, 03-04) of 18.2, which trumps any seasons Duncan or Dirk ever had, but his next highest were 16.1, 15.6, 14.9 and 12.9. Dirk's top five seasons were 17.7 , 16.3, 16.1, 15.6, and 14.6. The numbers are very comparable.
And Dirk is one of the, if not, the best offensive PF of all time, and he was at least decent on D (not as good as KG was offensively).