This is what the hitboxes look like.
Unlike most of Marth's other moves, the tipper hitbox on his dair has priority over the other hitboxes. This means that if you connect with both the tip and an inner hitbox, it will count as a tipper dair spike. Contrast this with Marth's fair where you can only hit with the very tip of it. Marth's uair shares the same properties as dair in the sense of tipper connection. The tipper hitbox has priority.
Something to note about your picture: When you're opponent is in hitlag, the animation doesn't match up with the hurtboxes. This means that picture will be slighlty misleading as to the true position of Peach's hurtboxes. She must have barely been scraped by the tipper hitbox.
