'|' is the command to pipe a selection through a shell command and replace it with the output, so I guess it's just a regular command on their system. You could use any other reflow or reformatting program you had handy. Kakoune in particular is really serious about delegating everything possible to the system.
Honestly, it might have just been a placeholder name for an arbitrary formatter. I vaguely remember old-school unix had a command for that, but none of them seem to be named exactly "reflow".
