If these reports determined that there was no evidence of something to be fixed (either the statistics were consistent with the statistics of qualified potential employees, or there were a good reason for them not to be consistent), I would expect GitHub not to care. And yes, if they continued to care about shifting the numbers further past that point, then you could accuse them of caring only about skin color when it's unjustified. But why should a technical company not measure metrics and gather data?
Again, this seems pretty straightforward for a technical shop... if your goal is, say, "We should not have features on GitHub for Desktop that preference one platform over another," then the way to get to that goal is you add testing processes that explicitly try out things on Windows, Mac, and Linux. Even if you develop your software with some platform-agnostic library (which is a perfectly reasonable way to achieve the goal), you test the software in a non-platform-agnostic way, precisely because you want to know if you got it wrong. You don't say "We're platform-agnostic" and remove the OS field from bug reports, unless you are 200% sure that your code is platform-agnostic even when things go wrong. If you see lots of cross-platform bugs, fine! You still leave it in there so that, one day, if somehow things are disproportionately broken on Windows, you're able to actively respond to that.