(Anonymous)
Dumbledore doesn't need to have guessed right about exactly what kind of rare occurrence actually caused the Harrycrux. Just throwing Lily's sacrifice into the mix could produce all kinds of odd effects (since apparently something about Lily's sacrifice is rare).
So, if Voldemort had been trying/planning to make a horcrux out of Harry's death, and Lily's sacrificial protection interfered... who knows what might happen just from that? Voldemort's soul wouldn't necessarily have had to be in tatters for a horcrux to be created, especially not if he was trying to make one anyway, and when he'd just killed two people, causing recent soul-shredding.
It may be tempting to assume that if Dumbledore reached a correct conclusion, then he must have followed the correct path to get there -- but he could easily have reached the right conclusion for partially wrong reasons.
Lynn
So, if Voldemort had been trying/planning to make a horcrux out of Harry's death, and Lily's sacrificial protection interfered... who knows what might happen just from that? Voldemort's soul wouldn't necessarily have had to be in tatters for a horcrux to be created, especially not if he was trying to make one anyway, and when he'd just killed two people, causing recent soul-shredding.
It may be tempting to assume that if Dumbledore reached a correct conclusion, then he must have followed the correct path to get there -- but he could easily have reached the right conclusion for partially wrong reasons.
Lynn