The issue here is that there is no universally accepted definition of 'factor'. For example, Wikipedia says that factors can be positive or negative. On the other hand, Manhattan GMAT's definition of a factor restricts them to positive integers only (p. 15 of their number properties strategies guide). The link here:
https://mathforum.org/library/drmath/view/63516.html features mathematicians debating this very question and concluding that there is no consensus.
So, Manhattan and Wikipedia both have legitimate definitions of 'factor', but Manhattan is a GMAT authority, so if they say that factors have to be positive, I would side with them for the purposes of this test. But honestly, if it were a real GMAT question, they would make it very clear in the wording of the question whether the answer should include negatives. There's no way they would write a question where the trick relies on knowing some particular definition of the word 'factor' that's not even universally agreed upon by mathematicians.
