I track my activities with a Garmin Vivoactiv3 and upload the data to Strava. My Strava data shows a graph of my cadence, but I don't have a cadence sensor. How is that possible? I have to wonder if that data is accurate at all.

Looks to me like the graph is VERY spiky and doesn't show enough consistency to be showing any attempt at determining actual pedal cadence. I realize that there's going to be noise, dropouts, and spikes, but there should be some more consistent data, especially on flats and climbs. I'm supposing that the app is too dumb to ignore the accelerometer inputs during cycling activities. And since you're sending the data from the original recording app to Strava, Strava itself is just showing what's in the data file it received.I recorded the ride with a 3rd party app for the watch called Dynamic Watch. Supposedly I can upload a map file to the watch and it's supposed to give me turn-by-turn directions, but it didn't work. I needed that level of navigation for a gravel ride in a rural area this weekend, but the DW app just showed me where I was in the past and not where I should turn. Technology fail!
It is possible that DW just records your activity but doesn't know if you're running or riding. I have NO idea what my cadence is, but I can probably calculate it by speed and distance because I ride a singlespeed.