That line confused me - I think we were expected to draw a lot of subtle inferences to figure out why it would make sense in this context.

On a side note, it is really jarring not to know everything Harry knows this late in the game. I always just read the third-person point of view as a matter of convenience, and accepted that we were fully immersed into the head of the current speaker. This distant outsiders' perspective ("I've done some research", "I have a plan") is making it really hard for me to draw conclusions.

It's also showing me just how much I relied on Harry running me through all the steps of some ridiculously complicated deduction. I wonder - does having a character who is both very intelligent and very honest mean that the reader has to be significantly less intelligent and active to follow along?