Raynn
Coroner
Yes, she did say that, and I think that's the way she remembered it, but it's not what really happened. Diana didn't literally leave, but in her mother's mind she figuratively left when the books fell on her, and she died. Then the mother's hoarding "blindness" took over, and in her mind her daughter was gone.I don't have the powers to re-watch the episode, but if I do recall, didn't the mother also say that after her daughter told her that she was okay, she left and the mother didn't see her since? Perhaps I am wrong.
The body was found in a different room than the books that made the impression on her head. Plus as Ray said, she didn't die instantly.
The mother was probably in a pretty messed up state by that time anyway, what with the forced clean-up, dealing (in her way) with what Alisa did, and then Diana finding the skeleton (I assume that's what she found), which could have contributed to her blocking out the body when she did come across it.