Thais have incorporated many foreign ingredients into their cuisine. For instance the chile pepper arrived only in the 16th century via South America. Peanuts are also a foreign adaptation. Even fish sauce they say has ancient Mediterranean roots. A cuisine is the taste preferences of a people-Thai, Vietnemese, American, etc.
Thai food is totally different than Vietnemese food even though the ingredients are very similar.
Even today Thai chefs are incorporating Western ingredients into Thailand's cuisine. An artist can paint on anything, not just canvas. Cooking should be a pleasure. The very best cooking is playful or childlike. Cooking is not a science- recipes are merely guidelines.