... don't paying Netflix customers already have access to the whole HBO back-catalogue?
Not here (Germany).
HBO isn't available at all on it's own. It's exclusively sublicensed (until the end of this year) to Sky which has a terrible terrible user experience and of course is another subscription.
Two days ago there was an announcement that HBO Max is to start in Germany in January. Let's see how that develops after the acquisition.
As a Netflix subscriber, that would be news to me.