The Inverted Song of Time doesn't make it possible to beat the whole game in a single cycle. You will still lose all of your bombs, arrows, etc periodically.
It's a similar situation to Mario 3D World. Giving the player "enough time" doesn't remove the presence of a time limit. It's still possible for someone to spend all of day 2 wandering around exploring because they don't feel like advancing the story, winding up short on time to finish the dungeon in day 3.
Not everyone knew about the Inverted Song, either. It's not like the game wouldn't let you progress without it. It has to be accepted as a nice reward for exploring rather than as an integral part of the experience, because if you treat it like everyone should have it then you are suggeseting that the game is poorly designed. If everyone should know the Inverted Song then it's an arbitrary addition to the game. They should have just made the days longer in that case.
That's not the case. It's a bonus for exploring. It's not to be assumed that everyone will have found it, just as it is not to be assumed that no one should have any trouble with the final boss because they can "just use the Fierce Deity Mask."
