The NSA leak weeks later that confirmed sophisticated and documented techniques for making hacks appear to come from somewhere else (e.g. by using existing known-Russian C&C servers) is unrelated, and it's apparently unfathomable to think anyone else would be able to use the same techniques.
Also, none of us have ever read history books, which are full of wars and witch hunts started over unverifiable claims by people in power that turned out to be mistaken (at best) or lies (at worst) to push their agenda.
So I'm not sure why you're asking for concrete evidence. They said it was the Russians trying to get Trump elected. Is that not good enough for you?