Actually the `windows-rs` team weighed in:

impl IRibbonExtensibility_Impl for Addin_Impl {

    unsafe fn GetCustomUI(

        &self,

        _ribbon_id: windows::core::Ref<BSTR>,

        out: windows::core::OutRef<BSTR>,

    ) -> HRESULT {

        log("GetCustomUI called()");

        if out.is_null() || out.write(BSTR::from(RIBBON_XML)).is_err() {

            return E_POINTER;

        };

        S_OK

    }

}

https://github.com/microsoft/windows-rs/issues/3832

Thanks for pushing on the issue! I've updated the blog post for GetCustomUI.