He's now said, that the engine is "totally unique and different", because it loads in 24x24 tiles rather than 16x16, and argues that the minor sprite changes make it completely his own style.
I encourage you to take the time to look up vanilla SDV screenshots (or use SDV's modding tools to decompile the tilesheets directly) and compare them side by side to the assets in my game, and you'll find that, while yes, pine trees, bushes, rocks, stones, stumps, grass exist in both games, they're certainly not just copied and pasted, and at best, I hope you'll recognize the effort I've put into distinguishing them from Stardew Valley's assets. 
