I think we can count it as an international that does not adhere to FIFA rules. I don't like the term "official". FIFA does not have a monopoly on what to call official or not. Even if 11 substitutes were made, it's still an international match. Even if a substitute comes back on, it's still 11 players against 11 from 2 nations.
Anyway, pretty dumb from Curaçao - they don't beat teams like T&T very often, so it would have given them some ranking points at least, which they could really do with at the moment.