I don't understand Draco's exchange with Lucius at the end of Chapter 7. Anyone know what's going on?

Here are my thoughts, which of course may easily be completely wrong.

Facts: 1) Harry states, "So during the Incident at the Potions Shop, while Professor McGonagall was busy talking to the shopkeeper and trying to get everything under control, I grabbed one of the customers and asked them about Lucius."

2) Harry states, "So you really are his one weak point. Huh."

3) Draco's letter to his father asks about Harry's "weak point" comment, Lucius's reply reads, "I would say that you had been so fortunate as to meet someone who enjoys the intimate confidence of our friend and valuable ally, Severus Snape."

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Conclusions:

• The wording of Harry's statement in fact 2 suggests that Harry's source put him on to the "weak point" assertion. The "so you really are" bit sounds like a hypothesis that has just been confirmed.

• A bit of a reach: Harry's source in the potions shop was Severus Snape, and Snape is allied with Lucius. If the first conclusion is true, then this would explain the speed of Lucius's response to Draco. That is, Snape tells Harry about Lucius, including that Draco is his only weak point, and then reports the conversation to Lucius; when Draco reports the "weak point" comment, Lucius understands that Harry has spoken to Draco.

• Snape is the potions teacher and hence might reasonably be found in the potions store.

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This is not very satisfying to me because Harry doesn't later recognize Snape as being someone he has seen before, and his appearance is quite distinctive to Harry. Polyjuice seems like a stretch.