LB -this is a chiddush to me as well. I think it’s probably a matter of semantics.
I never heard the term Noachide used until relatively recently. I think that the term Gentile used to be the only term used for non-Jews.
I guess now that the term Noachide exists, some people have redefined the word Gentile. However, I am not sure that the definitions you were given make sense.
I think that in order to be a Noachide, one has to keep all 7 Mitzvos bnei Noach and be doing it because G-d said to (there may be other qualifications, but I don’t know).
What if someone only believes in One G-d but doesn’t keep all the other 7 Mitzvos? What would he be then?