I would say yes, it is incorrect to call a beardless D'uccle a booted bantam. I trust what the breed clubs have to say about their breed more than what the APA has to say about a certain breed, so since the breed club has them as distinctly separate breeds I don't think it would be correct to...
You can read about the difference on this page: http://www.belgianduccle.org/belgian-duccles.html
The APA has the same standard for the two breeds, except for one has beard and muffs. A breeder here on BYC said the ABA has them as two distinctly different breeds, but I don't have an ABA SOP so I...