He's stated that people will be disobeying the law without any qualifying statement. Sweeping statements such as this are total bs. Everybody's situation is different. I know for a fact I'm not required to submit a tax return because not only am I a tax resident of another country, my DTA exempts pension payments from taxation in Thailand.
You won't find any text or law dealing with double taxation. If the OP is from a country with a DTA and has paid tax elsewhere, there's no tax to pay in Thailand. Unless of course someone is stupid enough to volunteer to pay tax, and I think many expats already fit this category. Thailand taxation is self-assessment, too easy