It is healthy to have skepticism concerning statements made by the government. This involves spy-craft bullshit, so it is really a matter of faith. I think the near past combined with other known disinformation incidents makes such statements less believable. You're not wrong to have concerns (but it is a bit inaccurate to say "no evidence", I would argue "incomplete evidence").

I loosely recall a quote from another country's intelligence minister (who had acknowledged state-sponsored attacks against his country) which roughly stated that spy-craft is better handled by spies than by politicians and the public. The nature of the business, I concur, does not yield well to an open discussion.

That said, the existing information available, even with its problems, appears well-founded at this time. But, even so, the correct response is that we must analyze this incident to secure our infrastructure. Had this issue cropped up with proper two-factor auth, I doubt the success would have been as high. So, it is reasonable to say that, despite the actor (which is really not that relevant on the internet), security concerns were not addressed properly and this result (as Trump, despite his nonsensical furtherance, pointed out) is from incompetence in security.

Perhaps they did hit the RNC too and maybe the release of information vis-a-vis the parties was released in an asymmetric manner. But you don't get to control what happens with information that leaks. You have to stop the leaks upfront.