That’s true.
Even the original forums dedicated to D3 during it’s development (prior to these here, that date back to September 2011) required a valid D2 or WC3 or WoW CD Key.
However, let’s not forget, that there’s a D2 section, and D2R section, and even the regulat D2 section required D2R pre-purchase, even if you’ve already have basic D2 and LoD linked to your account.
I truly hope this is not now Blizzard see people criticizing them… as trying to grief them… it would certainly explain why they seem to avoid their very own forums, and focus on social medias, that I can only describe as cesspools for feedback.