That statement is incorrect.
There is circumstantial evidence and direct evidence. The two versions are very different and as such, for the purposes of prosecution varying degrees of weight are placed on each.
Circumstantial evidence in this instance could be the scent of death detecting that a body has been present. Direct evidence would be the dog alerting to the scent of death upon which at a later date the body is found. Just because the dog alerts to the scent of death does not mean that the body was not there. It just carries less weight because it is clearly more difficult to prove which body was present.
Edit: Pathfinder just beat me to it but in much more detail 
I think you are confused.
"Evidence in the form of testimony from a witness who actually saw, heard, or touched the subject of questioning. Evidence that, if believed, proves existence of the fact in issue without inference or presumption. That means of proof which tends to show the existence of a fact in question, without the intervention of the proof of any other fact, and which is distinguished from Circumstantial Evidence, often called indirect."
http://legal-dictionary.thefreedictionary.com/Direct+EvidenceIn logical terms, direct evidence is a piece of information which (if true) commands a conclusion. If the evidence E is correct, Outcome O is proved.
Indirect evidence (circumstantial) is a piece of information which only suggests that (if true) a certain conclusion may be the case- it makes possible but does not command.
Dog Alerts can never be direct evidence even when confirmed by forensics.
F or instance in a certain case there was a dog alert, it was in error about the location, but a body had been present elsewhere.
It would not be possible for a dog alert to be direct evidence because there cannot be an impeccable logical connection between the information and the outcome, only a suggested, possible one.