If you live at least 180 days/year in Thailand you are considered TH tax resident. Assuring that you have no other tax residencies, when registering with IBKR you indicate Thailand as your country of residence providing address proofs. Then, when filling
W-8BEN form in order to get US-TH DTA WHT rate just tick the box in 6b field "Check if FTIN not legally required".
Everything seems to be perfectly legal, at least from the international broker and the Thai Revenue Department point of view, as plenty of foreigner/retiree residents in Thailand are invested in stocks with international brokers and receive US dividends withheld at 15% without any issue.
Could you please explain what your point is?