I just re-read that last night, and didn't see anything new. Perhaps he simply meant that without Snape helping all along and Narcissa lying to LV, things would have been tougher?
That strikes me as kinda lame.
Snape and Slughorn. Slughorn was dueling Voldemort with McGonagal and Kingsley at the end.
Don’t forget that Phineas led Snape to Harry and Hermione so he could leave the sword. He figured out where they were in the forest from his picture. I think that i s what he is talking about.