You are taking an analogy and turning into a actuality. No one ever said that the computers that control whatever system on the pipeline is the same or even similar system to a voting machine. The only similarity is they are both computerized and connected to the internet. After that they are as different as day and night.
Any computer system, especially one that is connected to the internet, has the potential of being hacked. That is just a fact.
So when conservatives make the analogy between the controls of a pipeline and voting machines they are NOT saying they are the same hardware, operating system, software or anything else like that.
They are saying that if one computer system can be hacked then why is it so hard to believe the another could have just as easily have been hacked.