Since there seems to be some confusion, I hope you don't mind me chiming in.
LimeZu is selling the assets under the CC-BY license, which allows you to use the assets (for either free or commercial projects) as long as you credit them.
The license you're talking about is CC-BY-SA, which also allows you to use the assets for your projects as long as they're credited. BUT there is another term added called ShareAlike, which means you need to distribute your creation/product under the same license. This is what you mean by sharing the source code right?
In summary, LimeZu's assets are under CC-BY, not CC-BY-SA, so you don't need to share your code :)