I am actually one of the engineers maintaining Uber's AthenaX open-source platform. As for now, AthenaX is still running on Flink 1.3.2 so it might be weird to follow the 1.4.2 release document. (We are working on upgrading to latest 1.5 release)
For your question regarding savepoints, AthenaX does not support savepoint natively at this moment. But we have a separated API WatchdogPolicy that you can customized. It supports monitoring / management APIs which will be called periodically. You can utilize that to trigger your daily savepoints. In term of how to do savepoint, I think the REST API  might be a good starting point in AthenaX case because we launch the job in detach mode.
Please let me know if this is helpful.