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.
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.
https://www.spinkube.dev/blog/2024/03/13/introducing-spinkub... is a really good blog post on Spin that talks about this.