The "kernel" of fuchsia is a microkernel, but when one talks about the Rust-free core of fuchsia, people usually mean the "zircon" component that encompasses both the microkernel as well as core utilities.
Around this zircon component they built a set of higher level components. They'd still be considered important parts of an operating system. E.g. things like a bluetooth stack.
The SDK you mention is for end user programs targetting the OS.
You're conflating two different things; outside the kernel, but inside of the OS, you can use Rust. They are using an increasing amount of it, as the other comment shows.
What you're talking about is the SDK for people writing applications to run on top of Fuchsia.
The documentation on fuchsia.dev heavily references Rust, and there is even documentation for usage with FIDL. I don't see anything explicitly relegating Rust usage to the kernel or even drivers. Seems like anything is fair game and "official."
