In the UK, the pension is classed as income (I suspect the same in most countries) and is taxed accordingly.
How exactly can/would "Thainess" come up with a different meaning ?
FWIW income from abroad was always supposed to be taxed, this is not a new thing "Thainess" has come up with.
However tax was only applicable if it was earnt in the same year it was brought into Thailand.