My (likely unpopular opinion)... if the goal is to make the Client Health Dashboard for Sinv "look healthy", I'd make one and only 1 rule for file inventory, something like....
look for this file: c:\Nothing\Nothing.txt and turn it on and deploy it to all.
My guess is that it's looking at summarization; and not taking into account that "never reported anything" is also a healthy scenario in your specific instance. If you turn on sinv, looking for nothing, at least the clients will report back that sinv completed successfully, so the dashboard will equalize. If the sinv rule is for a specific file in a specific folder (that doesn't exist), sinv will be fast.
Alternatively, if you don't want to cheat like that, then... do what I do. Don't look at the console for "client health"; make your own reports or look into one of the multiple 3rd party companies that provide custom reports.
or... if you want to continue to look at the Dashboard in the console, since you know that sinv isn't enabled, just know not to bother to pay attention to the Software Inventory section of the dashboard, since it is inaccurate in your scenario of "File Inventory not enabled".