Their Github account at https://github.com/prowriting looks boringly normal. I wonder what could have caused Github's systems to flag it and a customer service rep to decide it was not legitimate.
They also don't have any links to their GitHub pages on their site. This isn't how organizations normally use GitHub. One suspects they're just using it as a CDN without engaging with any of the coding or social features. Perhaps GitHub has some sort of trigger when particular resources get requested over and over without any referer? Why not just use GitHub normally? Failing that, why not just use a CDN?