What non-circularly defines a money-transmitting business?
> provides check cashing, currency exchange, or money transmitting or remittance services
says that Samourai is a money transmitting business iff they provide a money transmitting service. Part B:
> accepting currency, funds, or value that substitutes for currency and transmitting the currency, funds, or value that substitutes for currency by any means
says that they must be accepting currency/funds/value, which they don't appear to be. They only accept instructions from each user for how to spend funds to themselves.
They're exchanging currency. (SWIFT is just a messaging service, for example.)
Whether one can conclude that the people running the crypto mixer are engaging in criminal activity will depend on the text of the relevant statutes, though I'm pretty sure those will be fairly airtight.