That's definitely true at the major tourist attractions like temples, museums and national parks where there is an official policy of charging foreign nationals more.

If you point out the discrepancy, you are likely to get a shy or mischievous smile in return but you won't get an offer of a discount unless you have a Thai national ID card.

That said, unofficial overcharging by private companies seems to be less common in Thailand than it is in, say, India. I suppose it still happens every once in a while but I've tended to find bus conductors in Thailand very helpful and have fortunately avoided anyone like the person described above.