Actually, I seem to recall that he was working on it before getting hired by Facebook (unless there was a massive delay in the hiring to become known). I was following his excellent blog posts on the matter at the time.
Yes it was a fully working things before facebook. There has been a lot of improvement in both the core and cli. But the core innovations of zstd was well established before facebook. I was probably following his blogs (even though I wasn't a compression expert) for months before I saw the post about his joining facebook.