This is pretty impressive. I haven't looked at the SDK in detail, but managing to produce ergonomic APIs (semi-)automatically based on some service models for language as complex as Rust seems like a major accomplishment. I have most experience with the Python library (boto3) which is bit weird at places, but the Pythons dynamic nature is pretty good at hiding lot of that weirdness. Interesting to see how that works out for Rust where so much is static and explicit instead.
This also means that the possibility for multiple runtimes might not be that far fetched idea, if they can just make a code generator that spits out different flavors of the SDK.