It's okay to just identify as non-monogamous.
Non-monogamy is the umbrella term that encompasses the various types of ways someone can navigate intimate relationships and s-x beyond monogamy. Because this term is usually seen as too broad to cover - it has multiple labels beneath it that define the specific types of non-monogamy - polyamory, polygamy, sw-nger, monogamish, open relationship, etc. However, some people do not fit into any of those categories, may fluctuate between them, or want the flexibility that non-monogamy generally offers in its definition - so they label themselves as non-monogamous. And that's okay.
Is it incorrect?
Using non-monogamy as a term to broadly reference how someone forms their relationships and connections is not wrong. As stated before, some people do not identify with some terms that fall under the umbrella of non-monogamy. Some people may not find themselves polyamorous OR monogamish. If someone is transitioning into figuring out what non-monogamous practices fit them best, they may use the umbrella term to describe that. Some people have various types of non-monogamous applications in their lives. They may be an ambiamorous sw-nger, in which case, they may say they are non-monogamous. Someone may be bi/multilingual or be close to their ethnic background - perhaps all of those terms do not directly translate. Non-monogamy as a general practice and understanding of multiple intimate connections - regardless of the type - may be easier to translate and/or identify with directly. Non-monogamy does not always have to mean the umbrella term for all the types of non-monogamy. While it is okay to ask someone to explain what they mean and why they say they are non-monogamous, be aware that it's entirely valid for them to say that they are that.
What it doesn't mean...
It does not excuse nonconsensual behavior. Someone using non-monogamy to describe their infidelity or unethical practices loosely is not okay. Just because there is an understanding that non-monogamy can be interpreted in various ways does not mean that someone should violate boundaries or manipulate the people they are intimate with. If you are uncomfortable with how someone navigates their non-monogamy, you do not have to deal with them intimately. You can ask questions for clarification, but nonconsensual is simply nonconsensual at the end of the day.