In biology, that's what extraneous DNA means ~ outside of the organism's body. But from what I've seen, detectives seem to use it differently when analyzing crime scenes - I have seen it referred to (for example, on forensic files cases or other cases I've researched) as DNA that is supplementary, that is questionable how it got there. If not belonging to a second or third perp, there may be a worry of crime scene contamination or other. They want to identify all DNA at the scene to its source. It's not always possible and sometimes a jury doesn't care, but they should. I remember an article I read on the death of Irina Yarmolenko, which talked about there being "extraneous" DNA at the crime scene, not belonging to either men who were convicted. Will link if I come across it again.
Side note: One died and the other claims his innocence, multiple psychics have also said they are innocent and she was targeted by an acquaintance. A lot of people think they got convicted by DNA transference - i.e. maybe a cop looked at their identification, then touched the car or something. I don't believe these men did it or had any motive to. (But they are taking a wrong angle, imo, by trying to convince people that she killed herself).
Same with this case. It would make much more sense based on the brutality and that she fought back so hard that she and the perp(s) knew each other. Especially any crime that is in close physical proximity (i.e. strangulation) - most of the time it is personal. But extraneous DNA needs to be identified and sourced.
Note: Although, there is a slight possibility that they aren't using it this way or they said something else, but the media miscommunicated and paraphrased it a different way. Terminology is really crucial, but hard to determine what is what unless coming from a direct source.