Short of actually copying data that the Thai government considers its own property I don’t see a way forward on obtaining definitive park boundaries. They appear in Google Maps so the data is present and available somewhere but I have no clue as to how we might get permission to use it. We stumbled on a big collection of Thai topographic maps at the Chiangmai hackathon, maps that were actually done by a now defunct agency of the United States government. According to information on the maps, the U.S. government claims no copyright on them but the Thai government does. I tried to locate someone in the U.S. government that knew something about those maps but ran out of patience. Tracking down ownership and permissions through the U.S. bureaucracy was just too much work at the time.
I don’t know where those users you mentioned are getting their data but I would sure like to know their sources. Can anyone point me to a national park that has been recently added?