The tiny peninsula mentioned in the name is west from the Cathedral of Webs. If you get close to the edge of the hill there, you’ll spot it in the distance, between two tall trees. The trick with this one is that it is found on a higher platform. You need to find a way to jump on it from the platform above it.
Backtrack a bit from the road that leads to the Cathedral of Webs. Pan your view up and find a rock that is above the path. Use this rock to get to the highest platform, the one with the large tree on it. From this place, make your way down. The second lowest platform is where the tiny peninsula Skyshard lies, waiting for brave adventurers to share its wisdom with.
Sand Kingdom Moons - Super Mario Odyssey Power Moons