It's also a nationality and if you have that nationality, you are Thai.
Ethnicity is cultural characteristics/norms that we live by - and according to my sociology professor - ethnicity can actually change.
Anyone ever been called Farang-Laos by a Thai person before? ???? ????
I've met Asians who left Asia in their 20s to go West and by the time they were 60 you'd never know they even came from Asia. Ethnically, they were clearly Westernized, virtually 100%
And if you integrated in your teen years, you can be 100% ethically changed. I've seen this with Thai kids going West.
There is also a very old Sikh Thai community who are ethnically more similar to people in India, despite being born and raised here.
Culturally (ethically), they are as much Thai as a foreigner.
And don't get me started on Isaan people whose ethnicity is more Laos than Thai and southern Thais whose ethnicity is closer to Malay.