They weren't on any cloud provider previously. They famously had their own "metal cloud" of managed servers with everything being containerized and managed by Kubernetes. It seemed like it's worked pretty well, especially for their complex git operation tasks which had specific hardware requirements, but the official word is that apparently they're running into scaling limits with finding new datacenter capacity.
Yikes, that's worse, I thought the migration was at least a little politically motivated to reduce a dependency on a competitor like AWS or something. It's not exactly a great advertisement in any case to know that bare metal was more reliable for them than their own infrastructure when they now own it all the way through.