Here's what the spec [1] says about the sequence diagram aspect of the language: "[Ballerina's] abstractions and syntax for concurrency and network interaction have been designed so that there is a close correspondence with sequence diagrams. This enables a bidirectional mapping for any Ballerina function between its textual representation in the syntax described in this specification and its graphical representation as a sequence diagram, such that the sequence diagram fully shows the aspects of the behavior of that function that relate to concurrency and network interaction."
The two biggest things that are different about Ballerina in my view are the language abstractions for providing and consuming network services, and the correspondence with sequence diagrams.