Seizing eXch does really nothing to Lazarus. But okay, Lazarus laundered money through eXch but this money then goes somewhere, they cash it out, right? It can't be like all coins go to North Korea and North Korea withdraws it buy selling it on their own population. In the end, it all goes to banks and governments have access and all types of control over banks.
Governments are too incompetent to deal with any kind of cybercrime (not strictly related to North Korea), but it's very easy to deal with crypto sites by comparison.
If governments actually took hacking seriously, there would be less instances of successful hacks in anything, and more arrests of hackers.