Absolutely no one should doubt that Russia, and many other nations, have the capability to do what people are suggesting might have happened. The only question is whether they did, which would be almost unprecedented: the undermining of a mature democracy's electoral process by a foreign power would be a singular event. (Though, both the USA and Russia have done similar-ish things in the past, but often against immature democracies.)
Russia has shown interest in destabilizing the USA's electoral system throughout the year. The only question is just how far they are willing to go. We have the capability to determine whether they did this time. We should exercise that capability: if we don't, it will encourage our enemies (and our friends!) to do so in the future, regardless of what happened in 2016.