The project is focused on supporting Spin. But Spin supports a large amount of the WASI API surface and is headed in the direction of using WASI in place of Spin specific APIs wherever possible.
Hey SpinKube maintainer here. You're right that you could just run your Wasm (or Spin app) directly in a container and there often are reasons you might want to do that. But, SpinKube executes WebAssembly using a containerd-shim which means we avoid the overhead of starting a container and rather just directly execute the Wasm.