I've never seen that screen before, but I think the way itchio has set this up is kind of confusing. It's probably telling you about the Unity Tileset Collection unlock promotion because the asset you're looking at (Fences & Walls) is one of the required assets to unlock the promotion. Unfortunately, as you can see on the second screenshot, ownership of many more assets than Fences & Walls is required to unlock the Unity Tileset Collection.
That unlock promotion only exists to allow people who already own all my tilesets to collect the Unity package for free. It's not intended as a purchasing incentive. If you want the Unity Tileset Collection, it's much more affordable to just buy the Unity Tileset Collection than it is to buy every single tileset separately. Or maybe it would be cheaper for you to look at the list and buy the tilesets you don't yet own? I don't know what you've already got.