After reading the Tax Treaty between the US and Thailand, it specifically states that social security income and govt pensions can only be taxed by the US unless you become a resident AND a Thai national. A tax resident who lives in Thailand greater than 180 days is not a Thai National. See Article 20 and 21.
TLDR : Answer Summary
The US-Thailand Tax Treaty stipulates that social security income and government pensions are only taxable by the US unless an individual becomes a resident and a Thai national. Residents living in Thailand more than 180 days are considered tax residents but are not Thai nationals. There are discussions on implications of double taxation agreements and residents paying taxes based on income earned both in Thailand and the US, highlighting the need for professional assistance in tax reporting and exemptions.