In my mind, "abuse" connotes something longer term, or repeatedly. So perhaps they aren't able to conclude yet the timeline of whether it happened more than once. Perhaps it was repeated during the timeframe the perpetrator was in the house?
Perhaps I am also over-analyzing the word "abuse" vs. "assault" It's certainly possible Riddle thinks of them interchangeable. Sometimes it's easy to read into words that the person saying them didn't give a second thought.
Sexual Assault can either be Sexual Battery or Rape. Sexual Abuse is in a different category.
Bottom line is this, IMO:
If sexual wounds were discovered during autopsy of Amanda's body, and if forensic tests shows only Davey's DNA present on and/or inside Amanda, then you've got reason to question Davey as a POI related to the Sexual Assault.