Walking + Running Distance on iOS Activity App

Lately I've been doing a little more running than usual, so I've taken greater interest in Apple's iOS Activity App. Since I often run the same route, I've measured the distance on Google Maps and have found the Activity app to be highly accurate in terms of estimating distances.

I wonder if distance is estimated using data from its built-in accelerometers and gyroscopes alone or if GPS data is also used to optimize precision. To investigate, I checked my Location Services settings. There is an option to toggle Location Services on/off globally, or one can globally enable Location Services but toggle it on/off for each individual app. I discovered that the Activity app is entirely missing from the list of apps:

So does that mean Activity does not use GPS at all? Inquiring minds want to know.

2017-07-24 UPDATE: Today I turned off Location Services, so in theory the Activity app should NOT be able to leverage GPS, Bluetooth, or Wi-Fi to approximate my location while running. I then ran my usual 3-mile route, and the Activity app seemed to be just as accurate in estimating distance (walking + running distance: 0.3 miles before my run, 3.3 miles after my run), presumably from the accelerometer and gyroscope data.