I don't know if you noticed but the library also has separate borders you can use for the island. They are specifically there for custom additions.

Yeah, I noticed the borders, but didn't use them. I even made them invisible on all the furniture present in the image. I find them a bit odd, like why does the border go above the counter top? (Could be just me not used to see this kind of design on a border XD)
The one I'm more used to seeing have more of a rounded end. Like a Quarter Round or Half Bullnose.

Island tip: you can place cabinets back-to-back to create an island with doors/drawers. Fill in the counter top sides with side-borders.

Actually, I miss used the word 'island' furniture is more like a bar (not sure of the word). Doors/drawers on one side, and a place for stool or high chair on the other side.