Yes, you can be cheating yourself and others knowingly or unknowingly. It is all on how you interrupt your workout with Strava or Garmin. Strava is simply a tool. And just like any tool, you have to understand what this tool does otherwise there are unintended interpretations. The same applies to Garmin Connect.
Let's take a look at this workout of mine from June 6, 2020. Strava has two different times that it tracks, elapsed time and moving time. Moving time is the time that Strava detected motion. Elapsed time is the complete time it took to complete the workout.
For this run my moving time was 2:48:39 for a pace of 10:56. My elapsed time was 3:05:02. That is a difference of approximately 17 minutes. If the goal was to run 15.5 miles for the day, then what is my true run time? That would be the elapsed time of 3:05:02 for a pace of actually a 12 minute mile.
What happened is I stopped for breaks for various reasons. Nothing wrong with breaks, but again the goal was 15.5 miles. Breaks and all are included in that time. Does the clock stop in a race? No, it does not.
If you go for a run of any distance and you pause your watch as you walk or stop to take a break then your true time is the elapsed time because that is the time it took you to go that distance. If you use the shorter moving time then you are cheating yourself. Why? Because you couldn’t go the entire distance without stopping or taking a break. You are believing in an improbable time for yourself.
Another point is comparison of workouts. You can’t always objectively compare workouts with moving time, because during each workout you may be pausing your watch or stopping for a break in random amounts. This becomes an inconsistent variable that you are not accounting for. However, if you use the elapsed time then a comparison can be made on the progress of your fitness.
Now if you posted on Facebook about your moving time, you are giving people the false impression of your abilities. This is how you are cheating or deceiving others. Going back to my example. If I posted my moving time with a pace of 10:56 mile that would be misleading. Because in fact, it took 03:05:02 for a 12 minute pace.