It's actually sort of the other way round. C# has hardcoded syntax for async/await. F#'s syntax for async/await is a fully-general user-accessible mechanism.
It's absolutely not exactly the same; let! is only available within a computation block. If you want to return some value from the computation block and return to Functional land without having to pause the thread you need to use a continuation, which C# has built in syntactic sugar for in async/await and F# does not.
