I lived in a National Park for many years. Not one in the US - as the law there concerning house building prohibits building in most if not all circumstances. That may be the same with Australian NPs - I have no idea, but I do know that there are several NPs in the world that DO allow housebuilding within their regulation. Different countries have different levels of "protection" for the designation "National Park." It is something that is being reconsidered in several places, including Wales where I lived. In fact, the designation of National Parks (covering 20% of Wales) was being discussed in the Welsh Assembly as recently as this week - with a view to easing the regulations.
After making several agreeable trips to Koh Chang, I toyed with the idea of living there at some time in the future - maybe when children have finished their education, and other family ties are less important than now. As a result, I had looked at a website that provided what I assume is valid information for Koh Chang (and elsewhere in Thailand too, probably.) It includes a map showing the National Park there - some 80% of the island. In theory, that means 20% being outside the Park, though the terrain may not be suitable for housebuilding (or as attractive a prospect either.)
See:
https://iamkohchang.com/property/buyers-guide.html
I doubt very much that ATB is unaware of the regulations, and would have confidence in his integrity in their respect.