Never used or seen it before. Seems like PIO can do the exact same thing though with its filesystem tools (which supports LittleFS too in contrast to that tool), since in PIO you can have a data/ folder in the project and then use the tasks for buildfs and uploadfs (docs) to build the filesystem image (what they call pack in the extension) and upload it. Basically its a fancy interface to the underlying mkspiffs tool.
And since you can add new tasks to PlatformIO’s task list using advanced scripting, you could even add tasks which implements the missing minor features (list, download (from ESP8266), visualize).