Do you know what exploration even means?
An overworld is not the only means of exploration. Exploration is...just that: exploration without a hand hovering over you telling you what to do and being rewarded for it. Go to Lon Lon Ranch and you get a horse. You're not pointed in that direction, but you go anyways. The Biggoran Sword is an optional sword given only for doing a sidequest. There are multiple secret passageways that lead to Fairy caves where you gain new magic and abilities. You can practically do any of the dungeons as Adult Link out of order. Wanna do the Spirit Temple first? Go ahead! This is without mentioning that you have two time periods to explore. By setting things in motion in the past, you can do things in the future, like use peapods to find new secrets locations.
This is like saying Final Fantasy VI has no exploration, after all, the overworld is a barren wasteland, but that's irrelevant.
TP in comparison is far more barren as there's considerably less interesting thing to do in it. The only major secret I can think of in TP is that multi-floor dungeon and that dungeon sucks.