From the author in this thread: "Under the hood, Serverless UI uses the AWS CDK to build a CloudFormation template with a few assumptions built-in (eg. deploying a static UI, anything in /functions becomes an endpoint, etc)."
Yep! Diving deeper, the @serverlessui/construct package has a few custom constructs to facilitate the deployment. Additionally the @serverless/cli tool parses the project and configures a CDK App to synthesize and deploy.
Overall, it saves the manual setup and pre-configures a CDK application without any config for the end-user!