Great scene, Joel says we don't have to go on from here to the hospital and I almost didn't. I took a 2 day break after seeing more infected down that stairs after this beautiful, hopeful scene.
It actually made me depressed to see what was going on down there after this.
As to where the giraffes came from, you see a sign for the zoo on a wall just a few minutes before this scene in the game. I think the sign even had giraffes on it.