Absence of evidence is not evidence of absence.
4 people said he was there - people who were present at the campsite.
Search & rescue dogs tracked his scent to the reservoir twice in the few days after he disappeared.
That is evidence he was there.
There is no reason to disbelieve the last "sighting" by GGP.
Were his shoes seen anywhere at the location as it's highly unlikely the only footwear he had at the site was the oversized boots?