To be clear, you are suggesting that tax residents simply "hide" income such as US Social Security from a Thai tax return rather than documenting on the return that a DTA shields that US income from Thai taxation.
Unfortunately, in a world where Thailand's Revenue Department seriously pursues foreign tax evaders, ignoring Social Security income on your Thai return would be a bad strategy.
And there is always the possibility that your Thai marginal tax rate is higher than your US rate (or some US tax deductions don't apply in Thailand), so you would be required to pay the difference in Thailand. Hard to do if you don't report the income in the first place.
Note that I am writing about a hypothetical situation, there is still no guidance on whether Farangs who don't work in Thailand will have to file tax returns next year.