Funny enough, I already done that way before creating this post, due to wanting to explore the whole map without leaving tiny unvisited spaces. Because the vanilla visibility range is so tiny, there are loads of spaces on the map that are not visible on a normal playthrough. With that being said, when I hold down my middle mouse button on the world map, I can see that the island is, in fact, just water (seen in this video). I'm wondering about why I seem to have visited island before, though, because I don't have my visibility distance as high, as to see all the water of the lake shown in the video. And why would there be the quest marker, that's the biggest mystery (except when it's just a bug).