I'd like to see how you (or anyone here) propose to test it. This isn't science; it's a corporate press release.
Do we know the name of the "full, independent, third-party" investigator(s)? Do we know who paid for them, their methodology, and whether they reported to Horvath, giving her oversight?
Or are investigators the same, like software contractors?
You and I or 'tptacek probably cannot test it, but Horvath's lawyer may be able to sue to test it.
It would be very dumb of GitHub to state that there is no evidence if there is evidence that Horvath could turn up. (Granted, sometimes companies are dumb.)
Do we know the name of the "full, independent, third-party" investigator(s)? Do we know who paid for them, their methodology, and whether they reported to Horvath, giving her oversight?
Or are investigators the same, like software contractors?