What is the catch here? That seems good enough to run most things on.
In other words it's the same as every cloud provider: They hope you'll build a business system that uses enough of their proprietary tech that you can't easily move to another provider.
The autonomous database is Oracle's offering of their Oracle database - that requires no DBA or tuning and runs on Exadata infrastructure. They are offering it for free - because they want enterprises using existing on-premise Oracle databases, to move their existing DB applications to the cloud.
Oracle Autonomous databases comes with free Oracle APEX - which is a low code / no code toolchain.