What good would it do if China hacks Bitcoin? Bitcoin doesn't belong to any country.
Op does have a point here. Even if BTC is decentralized and doesn't belong to any specific country, it is still money which itself is the most important resource when waging wars.
Bitcoin is not fiat though. It isn't a currency that is endorsed by the government. If anything, fiat is the resource that would be used during war. You probably won't see governments paying contractors in Bitcoin during a war. The spending during a war would take hundreds of billions and I highly doubt Bitcoin in its entirety would be able to finance even a significant portion of it.
Attacking Bitcoin through 51% would be a waste of resources and even if they manage to attack Bitcoin, it would quickly become worthless and all their resources would've gone to waste.