MA: Did [Lily] know anything about the possible effect of standing in front of Harry?
JKR: No – because as I’ve tried to make clear in the series, it never happened before. No one ever survived before. And no one, therefore, knew that¹ could happen.
MA: So no one – Voldemort or anyone using Avada Kedavra – ever gave someone a choice and then they took that option [to die] -
JKR: They may have been given a choice, but not in that particular way.
J.K. Rowling's Exclusive Interview with Mugglenet -- Part One -- with Emerson Spartz of Mugglenet and Melissa Anelli of The Leaky Cauldron.
Voldemort counters J.K. Rowling with:
‘You all know that on the night I lost my powers and my body, I tried to kill him. His mother died in the attempt to save him – and unwittingly provided him with a protection I admit I had not foreseen ... I could not touch the boy.’ Voldemort raised one of his long white fingers, and put it very close to Harry’s cheek.
‘His mother left upon him the traces of her sacrifice ... this is old magic, I should have remembered it, I was foolish to overlook it ... but no matter. I can touch him now.’
Goblet of Fire - page 566 - Bloomsbury - chapter thirty-three, The Death Eaters
So how did Voldemort know about the magic invoked by a mother who dies, with love, for her child if it never happened before?
¹"that could happen" means someone could survive Avada Kedavra if his/her mother chose to sacrifice her life instead of stepping aside and letting her child be killed. At least that's the definition that I read within context of the quote.
I'm looking for an answer grounded in canon -- the books, Pottermore, or JKR interviews or writings are all fine, as are subjective answers within the spirit of canon. Thanks!