New crackpot theory, don't know if this has ever been discussed online at all, I came up with this myself during a re-read.
In the books Joffrey was killed by some sort of shadow baby/magic
If you had seen how Joff died . . . he fought, Jaime, he fought for every breath, but it was as if some malign spirit had its hands about his throat
What if unbeknownst to her, Cersei has accidentally stumbled across the real answer, and Gurm left the reader this clue.
I think it is entirely plausible. We're made to think Littlefinger conspired with the Tyrells to murder Joffrey. Preston Jacobs thinks that plan was way too impractical and Littlefinger meant to poison Tyrion so he could whisk Sansa away, but the flaw in that argument is Dontos knew they would leave the night of Joffrey's wedding long before she was betrothed to Tyrion.
I don't see why both possibilities can't be wrong, maybe Littlefinger lied to Sansa to make her more fearful of what he's capable of, maybe he did the magic himself, who knows. All I know is I had a revelation when I read that quote.