And after spearfishing exploits gained access to the DNC and DCCC computers, that access was used to install malware.
So, hacked. At that point the means of ingress became moot.
They detected that the malware was installed. They did know that they had been compromised. It's just that the horse was already out of the barn.
If they had been doing regular, strict monitoring of the systems they probably would have known earlier. They wouldn't have needed wikileaks to tell them.
Do we know if the RNC has had their systems checked by professionals, or what the results were? No. And I doubt we will, unless there is the same sort of involuntary public exposure that the attacks on the DNC systems got.
They aren't going to go to the media and say, "Hey. We got caught the same way." It wouldn't fit in with their talking points.