Not sure where you're getting the idea that it's a Bash completion extension from. It's a new shell (see name) that natively supports complex nested data structures, numbers, numbers with units, and so on. Compare with the classic POSIX shell model where everything is line-based (defined loosely), numerical operations rely on hacks, and splitting command output lines relies on hardcoded column indices.

It's definitely not bash completion. But on the jq note you can read JSON, and various other formats, directly into nushell's data model (a table) and just start querying it.