It was intended to be airgapped, but we're talking about a pipeline that is several thousand miles long, with many pumping stations and delivery terminals. All it would take is one of the SCADA systems at one of those locations to suddenly open a valve and dump petroleum out into the environment to cause a disaster.

Or worse - rapidly open & close valves in rhythm, and the water hammer effect (the inertia of the petroleum in the pipeline) would cause the pipeline to destroy itself. The repair costs would be astronomical - you'd naturally have to repair the damaged sections, but then also re-test all the welds to see if any had been weakened by the pressure pulses.

It can be already hacked but while power to the valves and pumps are removed then the SCADA system hacks can't cause physical damage.

I haven't seen any evidence that the "OT" side of their network was compromised in a way that would cause physical damage, a la Stuxnet.