Completely different cases. The GPL covers binary distributions as well, and if your source compiles to the mostly-the-same binary that doesn't make it different enough, and you're bound by the GPL to release the modified source.

I think the concept of derivative work is more important here. IANAL but I suspect there's a difference between copying someone's Skip List class (maybe modding it a bit) vs. writing your own even if your own compiles to mostly-the-same binary. Your own wasn't derivative even if the end result was very similar.