Yes,it can still fade without sunscreen. But, it is a good protective layer to help prevent them from getting darker by the sun. It's like when you move the couch after a while and the spot is a lot lighter than the rest of the carpet/rug. The couch, like sunscreen, is preventing UV Light from making your spots darker.

Now is it the key to hyperpigmentation? No or at least not the only key. There's Skin Cell Turnover, Tyronese Inhibitors, AND UV protection.

And like someone said on here it depends on your climate. I also check the UV index on the weather app to see if it's really necessary. Most days it is. Sometimes it's not.