I doubt that Facebook tries to hire Hack devs off the street. No one uses Hack except FB. They hire PHP developers and teach them Hack in boot camps.
The point being, you can hire good php devs.
The point being, you can hire good php devs.
Heck, I just read the docs of hack for 2 hours, and I already got a good sense of what the language can do. I worked with PHP internals, and a bunch of other languages in my career.
* as language we define the language, its syntax and semantics, not any other library or the ecosystem. And yes, the standard library is still just a library. Knowing a list of functions and parameters does not mean knowing a language.