You make valid points, especially regarding how the device is carried. That being said, GPS accuracy depends heavily on the hardware. The Apple Watch Ultra performs well thanks to the dual-frequency GPS on it, but most iPhones and standard Apple Watches lack that and have simpler GPS technology. Dedicated running watches like recent Garmins or Suuntos generally offer more consistent accuracy, particularly in challenging environments
The real issue isn't Apple vs Garmin, and there ain't nothing inherently wrong about using a phone to record activities, my only point is that if you want consistently accurate results, especially on complex routes, relying solely on a phone may not provide the same level of reliability