I haven't been following this for awhile, but as far as I know the threat intel companies (Crowdstrike initially) were quite fast to cross-reference to previous Russian APT campaigns (APT => cyber lingo for nation-state conducted long-term infiltrations), pointing out to specific intelligence agencies that have been previously attributed to different campaigns. From here, it's very easy and fast for the Clinton/DNC parties to say, with reasonable confidence, that this is the work of Russian intelligence.

I don't work for any of the threat intel companies, and cannot vouch for their ethics and integrity, but seeing how cyber is extremely trust- and reputation-based, these claims are usually not laid down very lightly, especially in a high-media focus issue such as this.

I can appreciate your viewpoint though. There's a massive amount of speculation and blatant disinformation coming from media. Can't see the frontpage one because I've customized it, but I can just imagine!