The Garmin InReach data logger is sending the Garmin data directly to Garmin's satellite, back down to the Garmin server to be presented on Garmin's webpage. You are not spoofing that. You seem to think that the way to spoof it is to get data pre-loaded onto the Garmin Inreach data logger, but that opens up other logistical concerns the conspiratorial team would have to confront, should surprise visitors show up.
Someone earlier already suggested one potential valid cheating scheme...a bicyclist is actually carrying the real tracking device, while the runner is in the vicinity in case visitors show up.
Visitors with binoculars could be a problem, IF they have valid realtime data to compare against.