Most accessible public land in the US allows you to camp in any one spot for a max of 14 days and forbids you from doing stuff like hammering a bunch of nails into a tree that will likely end up in a sawmill. Either this dude privately owns a nice big tract of undeveloped forest (Actually possible, 10 acres costs less than a Maserati in some places) or some BLM ranger is gonna be real pissed.

10 acres is also a lot more than you need for a treehouse! UK here, smaller woodland you could get for under £50k. Unfortunately you can’t legally live in it, I would rather be doing that instead of paying a mortgage.