https://archive.softwareheritage.org/browse/origin/directory... shows a "snapshot date" of "11 August 2015, 07:28:00 UTC" - any chance it was public on that date such that the crawler could have accessed it?
It is important to note that GitHub archive is not 100% accurate and there is over 319 missing hours.
Interestingly my public code with thousands of stars isn't in "The Stack".
Presumably github repo privacy state has an audit trail. This would allow GH to prove / disprove claims on any given repo easily. I hope a rep steps in to do so.