The multiple real-time Garmin + Strava data was conceived and proven out by Pete Kostelnick. Others may have helped him come up with what someone termed a "bombproof" documentation method. It has been called the Gold Standard for obvious reasons.
But the above is not all that was done. There are also a huge number of photos taken—Sandy posted, on Strave, more than 30 photos each of the last two days, and they are GPS spotted. Mimi does not have any GPS position photos she is sharing.
Pete K. also had many random people me meet sign statements that they saw him running along. I assume Sandy is doing the same—Pete K. is her documentation advisor, plus her ultra-expert guru—, but I do not know if she is.
Finally, Pete K. encouraged people to randomly join him at any point and time during his run (unannounced). Several did, and he always showed up based on the his real-time Garmin data, which you can open in any web browser on any smart phone. This makes it convenient for a random person to join and be forced to take a wild goose-chase looking for a runner. The burden is on the person doing the run, and not on the random follower.