They have made just over 300 Million off the initial BTC purchase, and as of 15 minutes ago, continue to buy BTC everyday.
They even have their own tracker, https://bitcoin.gob.sv/
I'd be surprised if DPRK doesn't already have access to the wallet and is just waiting to get the most out of it.
I highly doubt their BTC reserve is connected to the internet.
It's just a private key. You don't "connect" private keys to the Internet. You just have poor processes keeping it hidden while not losing it. I'm surprised you think El Salvador can do that.
In order for transactions to be signed with the private key, the device holding them has to be connected to the internet, or at least be able to interface with another device that is. It is highly likely that anything involving their BTC reserves requires someone to be physically present at some sort of facility and it is 100% not available for hackers to just siphon whenever they wish with a virus. If possible, it would at least require physical infiltration.