In the Garmin connect app there is the insights section where you can see how your performance ranks against other users. My question is; in the case of the runner's insight, are you ranked against all people who use the Garmin app or only those who actually run and use the running option on their watches?
A brief look at the charts gives you the answer quickly - if you sum up the %-values of all columns, it results in ~100%. So for example if you take the chart for number of running activities, it starts with 1 activity/month, and in my age category 15% of users log one run per month. All columns together give ~100%, which clearly means users not running at all are not included. If they were included, then the sum of all columns without the 0 value, would be either much lower than 100%, or there would be a column for the value 0 with a pretty high percentage.
The help which opens when you click 
next to Insights also confirms what trux said:
The implication is that any sport-specific insight (e.g. running or cycling) only includes data for people who actually track that sport with the Garmin (as opposed to all Garmin users)
Also, if *you* look at an insight where you don't track that sport, the text about how you rank compared to other users is omitted (and there's no orange section of the graph which normally indicates where you would fit in), which implies that you're not included in the data set (and suggests that neither are *other people* who don't do that sport).
For example, I run but I don't cycle or swim, so I have the text which compares me to other runners, but not the text which compares me to cyclists or swimmers:
If Garmin included non-runners in the running graph or non-cyclists in the cycling graph, you would expect a very tall line on the left side to show all the Garmin users who never run or cycle. And if you don't cycle (like me), you might expect to see text like "you cycle farther than 0% of Garmin users."
