The hell is that weapon that would necessitate all of this? The gravitas and the earthquake made me think of a nuke, but they said that they'd need to manufacture "rounds".
My guess is they weaponised the plague into a biological weapon.
Until now the plague was quarantined to the island so it's delivery to the mainland could be devastating to the un-immunised mainland population while being of limited risk to the already exposed islanders.
Like nukes, just the existence of such a weapon would be a powerful deterrent against the oil tanker blockade and proxy warring, and would put the island on equal ground to enforce independence and sovereignty in the long term. Anything less wouldn't match the gravity that both governments are clearly treating it with.
'Rounds' could refer to delivery via super heavy artillery, it's possible the factory complex is actually meant to support the production of something like a railway cannon with range to reach the mainland. Alternatively rocket assisted artillery (like the V1) should be viable for the established tech level.
Presuming that range is possible, ballistic delivery should be uninterceptable at the current tech level, especially presuming the mainland has a technological/industrial edge on land and sea that makes
alternate delivery methods unviable.
Alternatively 'rounds' could be an imperfect translation, or intentional code to maintain secrecy?
I was thinking they're developing a reliable mass-produced infantry weapon like their version of the AKM.
I certainly don't think it's anything small or conventional since we have seen plenty of precision and automatic rifles, tanks and aircraft, etc being passed around in the civil-war. There's not much room technologically for anything conventionally groundbreaking that would trigger a desperately rushed invasion like this has.