"They don't say how; why is that?"
I believe we've had other articles explain, at least in part, why that is.  A complete and clear explanation would involve outing some sources that they don't want outed.  There are reports that the hacks are ongoing and that the software used has a "unique signature" so if the sources were outed the Russians could made it harder to detect if the software's signature and how it is being tracked is identified.

"this doesn't excuse what the DNC did."
A bit of wagging the dog?  I don't see this past event being more important or even as important to discuss at this time so bringing it up now seems to be an attempt to deflect or obfuscate.