Since many reptiles keepers have kept and bred many of or native species over many years, I would suggest that they would indeed have a good idea on the longevity of those species. One of the biggest clues is when they suddenly stop breeding because they are simply too old. Secondly some people have indeed had their dead pythons that they had suspected to have succumbed to old age, autopsied and sure enough, arthritis type conditions and other evidence of old age.
My conclusion, 100 years for any Australian python is absolute BS.