It's long been assumed, with good reason, that Lee had a lot of material lying around from when she was writing Mockingbird. Lee's issue was never with simply producing material, but with shaping it into a compelling overall structure. Lee relied heavily on the support and advice of her agent and her Lippincott editors in composing the book; it's well-known that it was heavily revised and the whole process was drawn-out and complex.
It is not plausible -- though I suppose it's possible -- that Lee just had a polished, complete, instantly publishable manuscript at the bottom of a drawer somewhere. Given what we know about the editing process for Mockingbird, and about the awfully shady circumstances of this announcement, I'd need lots more transparency about the provenance of this "lost" novel before it doesn't come across as kind of scummy.
Thers I am the author of a book about Harper Lee; would be weird if this got weird.