"Heroin" is "inherently distinctive" in that it's a totally made-up word; it isn't a purely "generic" term like "Windows" or even a "descriptive" term like "Whole Foods". Even in German. The drug isn't literally extracted from heroes, it doesn't contain heroes, nothing like that. The reference here is to the spectrum of https://en.wikipedia.org/wiki/Trademark_distinctiveness. Even if most of the caselaw cited there is anachronistic in this case, many of the principles already existed at the time.