it is not just their citizens tho is it, it is their tax residents. a french citizen tax resident in the uk will have to pay tax on worldwide income just as a uk citizen tax resident would. i agree that this will not likely go ahead in thailand but it certainly is possible.
how do you think the uk or usa gets this information and taxes worldwide income? it is still a proposal at this stage but it certainly is possible and doable imho. but as ever i will wait and see what officially happens or doesnt