Who are they? Thai government or Thai people?
The "Thainification policy" has done some damage to some people's knowledge about the "actual Thai history", which is taught at a university level, free from the influence of the government rubbish version taught at school.
Trust me, social media and this feud with the cambodians over history has led many young Thais to question the history taught at school and done their own research on the Thai history (written by academics, not by government) and discussed it on various social media platforms. The outlook on this will be different in the future.
As with cultural appropriation, i personally think that it really depends on the extent to which the said culture, practice, object, etc. depart from its original form. When the extent is very significant, it means the said X has gone through alot of development and idea generation, I don't think it's wrong to say that such object is Thai. For example, a burmese invented a pair of shoes and they look a certain way, if a Thai came up with a pair of shoes which look totally different from the shoes made by the burmese, i don't think it is wrong to claim it's a Thai style pair of shoes. And the burmese should not call it a pair of burmese shoes. Of course, we need to acknowledge that the burmese is the one that invented the first pair of shoes.