Generating assembly from C is an implementation detail, and many C compilers don't do that.
On the other hand, Dhall really is a YAML generator: the available tools allow only one-way conversion (in particular, there is no interpreter/library to ingest Dhall from the configured application itself).
Not true. Perhaps this needs to be highlighted more prominently on the homepage. There are directly language bindings for Ruby, Haskell, and the JVM with more on the way.
As for Python, that's because the Python interpreter can be embedded in an app.
That said, Dhall is exposed as a Haskell library, so if you're writing a Haskell app you could consume Dhall directly and skip the YAML. https://hackage.haskell.org/package/dhall-1.24.0/docs/Dhall-... shows examples of this.