> If hackers were (hypothetically, surely) to cause a liquid natural gas plant explosion, is that less terroristic than causing a truck, [...], a wastebin, a church, or a policeman's pub to explode?
If there is a war going on, then yes definitely. A LNG plant is a strategic target, same as the data center. The truck could also be argued as well. But a waste bin? What strategic aim is there in blowing up a wastebin, other than instilling fear in the local population? If the sole strategic impact of an attack is the terror it causes, that is terrorism. Of course you can get into shades of gray, like the truck. Maybe the truck was being used to transport materials for the war, or maybe not.
Ever since before WW2 the legitimacy of a target in war time didn't matter anymore. Personally I go further, if one party can use, e.g., cruise missiles to blow up stuff the other party should have the same right blowing up stuff. Even they use other means for lack of cruise missiles. As soon as you explicitly target civilians and non-combatants it is terrorism or a war crime, regardless of the means. We owe it to WW2 that strategic bombing and unrestricted submarine warfare are considered legitimate strategies, simply because both sides used it and the victors wanted to avoid casting and cloud over their tactics and people like Harris.
The wikipedia page for strategic bombing is where I pulled that Churchill quote from. When strategic bombing specifically targets a civilian population with the intent to cause terror, by Churchill's own account, that is terrorism (the term 'terror bombing' has been used by historians and commentators to describe the practice, and I think it fits.) Both sides of WW2 used terrorism extensively. They normalized it, but normalized terrorism is still terrorism.
"bombing of German cities simply for the sake of increasing the terror, though under other pretexts"
If there is a war going on, then yes definitely. A LNG plant is a strategic target, same as the data center. The truck could also be argued as well. But a waste bin? What strategic aim is there in blowing up a wastebin, other than instilling fear in the local population? If the sole strategic impact of an attack is the terror it causes, that is terrorism. Of course you can get into shades of gray, like the truck. Maybe the truck was being used to transport materials for the war, or maybe not.