I agree with you, but some don't; I lived in Cambodia for a while, and there were/are "ex-pats" living there with Khmer wives and children, who speak perfect Khmer, work in Cambodian companies, have been there for 20+ years, and still refer to themselves as "ex-pats".
But I think this is might be cultural about the host country. Cambodia is blatantly racist, "Cambodia is for the Khmer people" is a thing there. Any non-Khmer immigrant will never be considered Cambodian, always a "barang"(foreigner) no matter how long they live there. This isn't true of NZ, who would happily consider you a Kiwi if you got the citizenship and lived there for a while, no matter your race or origin country.
> Cambodia is blatantly racist
In my experience, there is nothing special about Cambodia in this regard. You could substitute any poorly developed nation, and you would have similar results. If Cambodia had a GDP per capita similar to Japan/Korea/Taiwan, they would "suddenly" become less racist because they would be much more concerned with economics rather than ethnicity/religion.
An expat is someone living outside (ex) of their homeland (patria, technically fatherland rather than homeland if we are pedantic). All immigrants and non-immigrants [1] are expats, by definition, no matter the connotation some people have decided to give the word.
[1] Non-immigrant is the administrative word used by the United States to designate people that are supposed to leave the country rather than settle in the US, usually on training/work/investor visas.