If this group is so powerful, then why would they use mixers that are under the supervision of the authorities, and not invent a much more effective solution for money laundering - for example, through the banking system that launders hundreds of billions every year, and at the same time we do not hear that the authorities have closed any big bank.
Probably because of heavy sanctions that are in place against everyone and everything that is connected with the ruling power of North Korea. Lazarus Group is always mentioned as a state-operated hacking team. If that is true, it would make sense that they use Bitcoin and other assets outside of the traditional banking system. They would be fools to use tether and similar centralized tokens that can be easily frozen. I just read that over 100 Ethereum addresses were frozen over the weekend after a new Tether policy update. There wasn't a lot of money, but still.