alt Hacker NewsIt's a descriptive label of the general lineage the designer considers their font to be in, not an ISO spec.
A little bit of looseness here is a pretty strong signal for careful font design actually. It means the creator considered some other factor (in this case char disambiguation) to be more important than rigid adherence to the category.
Or, to put it in a way that doesn’t make you sound like a complete ass: serif characters are the exception, so this is considered a sans-serif font.