The C# implementation is clearly an attempt of putting type-safety over exactly the same implementation JS promises use. Done because MS wanted to port the same behavior.
- the C# implementation predates even Promises in JS, so it is not "the same implementation" and your implication that C# was inspired by JS as opposed to the other way around is false. More background: [0]
- Typescript works fine with the JS implementation so any differences aren't for type safety reasons, but largely because C# has a multithreaded event loop unlike JS
Also promises (or "futures" as they're called elsewhere) aren't unique to any language. They're used in lots of places that predate both C# and JS's use, for example the twisted framework in Python.
You're implying people went from writing C# to JS code willingly and not just wrote it but went on "improving" the ecosystem and I just don't believe there are people insane enough to do it willingly so "it was invented separately in both instances" is far more likely.
I can't tell if you're talking about the language designers or users.
For language designers I point out in my other comment that Anders authored both C# and TS. TS' influence on ES6 is documented publicly.. heck TC39 has an open proposal to add type annotations to JS now!
As for users, any dev that touches frontend has to write JS, unless you're purely a mobile or desktop shop (even then, there's electron). So yes, I think tons of folks willingly write C# and JS/TS. I'm certainly one (though write more Python than both these days). Was I an early adopter of async in Python because of my familiarity of it in C#/JS? You bet I was. Maybe I'm "insane."
I see this repeated everywhere in this thread. async/await originated in C# not JS.