They didn't know if a sexual assault took place at that news conference. The autopsy hadn't been done. Also, wasn't the autopsy inconclusive with regards to sexual assault (I honestly can't remember)?
After autopsy, all that could be said was that evidence of sexual assault was not obvious, and there was an opinion that she could have died as a result of an external cause.
I have no idea how anyone could conclude, on that basis, that the murderer was familiar with the victim, and the victim was specifically targeted by the murderer. External causes in homicidal violence are rather common, particularly in victims of sexual assault. Absence of evidence of sexual assault certainly doesn't rule out sexual assault. Absence of clothing indicates sexual assault.
Essentially, the autopsy suggests to me that there was an attempted sexual assault and it was a soft kill ... one that wouldn't draw a lot of attention like a gun, and wouldn't make a big mess like a knife.
"There were
no obvious signs of sexual assault, the medical examiner reported, and there were no drugs other than caffeine in her system. Ethanol was present, but the medical examiner concluded that was likely created by the decomposition of the body.
"Based on the history and autopsy findings, it is in my
opinion that she died as a result of external causes, homicidal violence, most likely asphyxia by strangulation," Dr. John D. Butts wrote."
http://www.wral.com/news/local/story/3638830/
A case that comes to mind is that of the murder of Meredith Kercher (British Erasmus student in Italy). Some of her clothing was removed, but there was no evidence of rape.