This is true...
To me, the "footnotes" are the people who are mostly forgotten. The ones who aren't studied at all - the ones who only come up in a sentence or two when dealing with another topic or person. So... in my opinion, the British royals alive today that will be "footnotes" are the Gloucesters, Kents, Phillips, and York and Wessex children. They're people who aren't likely to ever be studied themselves and will only come up in studies of their parents, grandparents, or the Queen. Diana, in contrast, might not have any more of a lasting impact than the Queen's cousins or most of her grandchildren, but she's a figure who's going to be studied in her own right and along with her ex-husband and his second wife. The scandal of her marriage may not have had any lasting impact - and beyond whatever impact it has on Charles, Camilla, William, and Harry, I don't think it has any significant lasting impact - but it's something that has captivated the imaginations of people and as such has, in my opinion, ensured her legacy. She'll be studied by academics because of her marriage, not her impact. She'll be remembered in pop culture for the same reason.
Consider... Madame du Barry. She was a mistress of Louis XV and was not herself a significant character - she had no interest in politics, didn't birth an heir, etc. She had no lasting impact outside of her life. Yet we still remember her and people still study her - Wikipedia lists sources on her as recent as 2005. History continues to care about who she was not because of anything that she achieved but rather... well, because of who she "did". I think that's the kind of legacy that Diana will have. It's not about the things that she did that changed the world or impacted a significant number of lives, but how she captivated the attention of people.