The problem is how do you prove you didn't base your ideas on what you saw? If you never saw it then its easy to say you can't have copied it.
Are there any verbatim copies of the original in your project. Did you sign an enforceable ip ownership or non-compete agreement with GitHub about trade secrets learned while working on their source code?
If the answers to those questions are no, there is a very long, very steep, uphill battle facing GitHub.