Walking through the woods with my compadre, Mario, I see a man coming towards us. I turn to Mario and say, "Do you think that man's dangerous?" and he responds, "No es necesariamente una persona, Gohma". I turn my head, slowly, towards him and mutter under my breath, "...aliens Mar--" but before I can finish my sentence my compadre is shot up in a beam of light!!
It could read "she/he/it" is not necessarily a person. ES is the conjugated form of the verb "SER" meaning he, she or it IS. The "una" in the sentence is the feminine adjective related to the feminine noun "persona." If "persona" in the sentence had been a masculine word like "carro", for example, the sentence would read: No es necesariamente un carro.
As droma pointed out, "es" can be translated as he is, she is, you are (usted es - formal you) or it is. The context would be clearer in a longer passage. Whenever you think duoLingo has overlooked a correct translation or made another error, you should use the "Report a problem link" to get the duoLingo staff to consider your argument. The report a problem reminder is not meant as criticism because this forum is a great place to get information from others.
To make sense of why the es in this phrase is not speaking of an implied person you need to recall that ser describes "essence", so what we are drawing with this phrase is an equality of two different characteristics (x=y). As this phrase reads on its own, the referent cannot be intending a person (S/he is not necessarily a person) and make sense because a person is always a person (x=x). Because what it actually is is unknown and possibly not human then it is understood to be generic and genderless. This is also the logic of the English phrase and why it uses the ungendered, generic, and emphatically non-personal pronoun "it".
This is only true, however, with the given phrase. As other comments have suggested, by adding a just little more information we can make the referent a he or she. All we need do is qualify the type of person and we open up the possibility that the referent is a person after all.
- El fanático no es necesariamente una persona que comete crímenes
- Una persona ociosa no es necesariamente una persona improductiva.
- Una persona no tiene por qué pertenecer necesariamente a la especie humana, aunque no se conozca actualmente ninguna otra especie en el universo cuyos individuos sean personas y, recíprocamente, un ser humano no es necesariamente una persona.
As you can see, you need to be qualifying the type of person, essentially saying that "Person of type A is not equal to person of type B" (and in the last example "Person is not necessarily equal to Human") for a personal pronoun no make sense as a referent.