I'm inclined to wonder if Harry's ability to block the killing curse with his patronus might be enough to clue Quirrelmort in to how it works. Possibly any collision of their magics would have produced a similar effect, but if not, it still makes thematic sense that the patronus would be able to block a killing curse, since it represents defiance of death. Given that Harry has already told him that he made the patronus by thinking about the eradication of death, I wonder if Quirrelmort might have enough information to piece together the nature of Harry's revelation.