I think there should be a sensible threshold somewhere.
Logically, if it is eaten regularly (almost daily in some cases, lol) by enough Thais then it's part of Thai cuisine (e.g. papaya salad, southern curry).
This cannot be said for tacos or bangers n mash.
Very few Thais have ever even tried those, nevermind adopted them in their regular diet.
Let's try to be sensible here.
To say papaya salad isn't Thai food, is like saying everyone in Isaan isn't Thai.
Isaan people are Thai. They eat papaya salad in their regular diet Thus, it has become part of Thai cuisine. It has Laotian roots though and a longer history in that country.
Same can be said for the southern curries and chicken over rice etc. etc.
By your logic, you might as well say chilli peppers aren't part of Thai cuisine since they didn't naturally grow here. They were brought here by the Portuguese and Spanish. The Thais then started cultivating them.