rofl
They could start charging 15% tax to all the Chinese, Taiwanese and Hong Kong landlords who own multiple properties in Thailand. That would be the EASIEST thing to do since the government could just go to land department and see who the owners are.
OR even easier just track the monthly rent payments foreign landlords have set up with Bangkok Bank where both Thai and Foreign tenants pay them.
No they can't even do something as simple as that.
By your interpretation at least all foreign landlords should have been paying tax on their rental income when in fact none of them are.