TIL that that the selection between public, followers only, or private in the end ride window does not translate to any third party apps to which your activity is being saved. In my case, my activity writes to Strava. I wasn’t interested in publicly publishing the results of my ramp test to Strava, so I selected private. My friendly Strava followers were quick to give me kudos. I was quite shocked.
I recommend the Zwift design team help users better understand the limits of that privacy selection.
Same applies to all integrations, not just Strava.
In the meantime, you could try contacting the author of activityfix.com and see if he could add changing of privacy settings to his rules based app. I currently use the app to change the bike type recorded in Strava from my default bike, to something else based on ride type (“virtual ride” I think is what Zwift send to Strava), so adding an option to change activity privacy to private/followers etc should be possible, if Strava let him access that in the API.