Those docs are out dated. I really do hope jps either takes them down, or adds some kind of deprecated banner (https://forum.sublimetext.com/t/request-to-take-down-st1-documentation/9290/1). Here are the updated docs. Hopefully they will be more helpful.
docs.sublimetext.info/en/latest/ ... stems.html
The command being run is just an array of strings, so you should be able to add your flag there.