You are free to believe any Thai law expert or smart-ass official interpretation you read here and there but there is only one thing that is right: the Thai law. And it says:
Taxpayers are classified into “resident” and “non-resident”. “Resident” means any person residing in Thailand for a period or periods aggregating more than 180 days in any tax (calendar) year. A resident of Thailand is liable to pay tax on income from sources in Thailand as well as on the portion of income from foreign sources that is brought into Thailand. A non-resident is, however, subject to tax only on income from sources in Thailand.
This remains valid until amended.
https://www.rd.go.th/english/6045.html
So unless you come back with a Thai court judgement or case law to back you up, any statement saying that "A non-resident could be, in certain case, subject to tax on income from foreign sources" is irrelevant.