not true. My home country UK has a DTA with Thailand and I would in some circumstances be required to pay Thai tax on foreign sourced remittances which has already been taxed in the UK.
You automatically become tax resident after 180 days in the Kingdom in any given year. That means you become subject to Thai tax law, that is all. No automatic TIN or tax filing required. You will need to get a TIN if you are tax resident AND have a tax liability in a given tax year. Tax liability means you have domestic income or have remitted foreign sourced assessable income above minimum thresholds. What classifies as assessable income can be found on the TRD website and there is an English translation of the Revenue Code there too.
just because you become tax resident it does not mean you automatically have to file taxes. You can legally reside in Thailand without paying any Thai tax or filing a return. It all depends on your personal circumstances.