They were originally on their own datacenters + huge amounts of burst and ancillary stuff in AWS, the internal push to move away from "the competitor's cloud" after the acquisition was huge and entirely stupid. I'm ex-GitHub and was one of the annoying people constantly saying GitHub should only move where it was provably the best option for GitHub - the third best major cloud provider is likely never it on merits alone.

If this story is true, it's good that they finally realised that GitHub's performance and availability mattered more than using Microsoft's products. It would mean someone finally came to their senses rather than forcing a wholesale push to Azure - but I bet they still want to have it both ways even if they concede some AWS now.