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.
That is because they cannot just use any mixer, as nobody is sure whether they, and which ones, are FBI honeypots.
A lot of their stash is in other coins besides Bitcoin. That precludes using mixers for any of them because the ones on
bitcointalk only work with Bitcoin.
Also they need to be sure that the owner is not going to sell them out. For example (and this is only theoretical), maybe MixTum is forced to hand over Lazarus bitcoins over to FIOD or some other government should they ever get any.