it’s a written rule but only enforced if you are coming from a very poor country or you appear to be lacking funds to enter the country. It’s extremely isolated. For example, I have entered Thailand well over 100 times never been asked. Same for other people I know.
nobody knows how things work here in Thailand been using my Dutch driving license for 7 years been stopped by police and they could care less. all these people quoting laws that’s not how it works here.