If there's one thing large organizations are good at, it's managing risk. And if you read their post, they've done this.

They're running both source control systems in parallel, switching developers in blocks, and monitoring commit activity and feedback to watch for major issues. In the worst case, if GVFS failed or developers hated it, they could roll back to their old system.

Again, to my point above: there's a cost to doing this but it's negligible for very large organizations like Microsoft.