Let's look at the last part of this comment first. 'Hard evidence', is exactly the testimony of someone with direct knowledge of a crime. A witness, in other words. This is known as "
direct evidence". There is nothing which precludes a witness from
also being a participant. Very little other than witness testimony of the crime itself (such as Knox's statements and writing) is legally regarded as direct evidence, and not all of that.
Indirect or "
circumstantial evidence" would be pretty much everything else, including things like fingerprints or DNA, or any expert testimony relating to them. Many people are confused about this, and suffer from the misapprehension that it is difficult to convict on circumstantial evidence. It isn't. Most evidence is circumstantial, and many cases are proven as a result.
In the process of investigating a crime standard procedure is to interview as many people as necessary. This process normally begins with those people closest the the victim or the scene until further evidence leads elsewhere.
There is absolutely nothing the least bit unusual about some of those interviews leading to an increased interest in the people being interviewed, nor to some of those people making the transition from a source of information to a person of interest, and then to a suspect.
In fact it is far more the rule than the exception in cases where guilt is not obvious at the scene. I don't understand why you feel that this case should be different than normal. I certainly don't understand how you can characterize the practice as some sort of technicality.