To not support the FLUSHALL command suggests that Azure is the goal with the project.
Either way you can see contributions to add more commands here: https://github.com/microsoft/garnet/pulls?q=is%3Apr+add+comm...
With that said, I'm slightly skeptical of/worried about Garnet but the reason is different - it received a bit too much hype soon after going public and I'm concerned it will be subject to corporate politics that often plague projects like that.