Indeed, which means they can not tax any other form of social security, Thai, foreign, whatever as it would be a breach of 'non discrimination' - that's the argument being put forward as far as I understand it.
It's a new angle and that's the first time I heard it, will be interesting if it plays out.
We already know some people have been turned away from filing tax returns, even when they tried multiple times - because they submitted pension income - which they thought would trigger taxation - perhaps this is the reason why.
I guess we will all know for sure in a year or two.