I don't think so. To qualify as a money transmitter in the second scenario, one would need to both develop the mixing platform and also profit from the associated money transmission.
It's arguable that a market-maker does neither. They are profiting from the bid-offer spread, not the mixing activity:
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You would see the same distinction with an exchange. A market-maker on an exchange doesn't pocket trading commissions whenever trades are made -- the exchange operators do. That's who FinCEN is targeting, the ones who build the platform and are profiting from it.
That's certainly true, mate. FinCEN targets central operators who profit from market trades, money transmitting services, etc. But I find it odd to see the SEC targeting the Etherdelta developer after being a decentralized exchange with no central operator. If they've still enforced securities laws on a decentralized exchange, they could easily make decentralized mixers' developers accountable too. After all, the government would not want people to use crypto in order to obtain financial freedom. Once they see Bitcoin and other cryptocurrencies as a threat to the existence of the current monetary system, you could expect something like this to happen in the future.
We cannot discard the possibility of governments making developers accountable for decentralized exchanges, mixers, and other crypto solutions. The US government has been the most strict one when it comes to crypto/Blockchain regulation. They've taken down the mixer in question and could easily shut down others over time. Once people start using non-custodial Bitcoin mixers like Wasabi Wallet and JoinMarket, you could expect opposition from the US and other worldwide countries. The main excuse from these entities will be that decentralized mixing solutions open a pathway for criminals to launder money and evade taxes at will. While we know that Fiat is used more for this than crypto, governments don't see it that way.
In the end, this will be a never-ending battle between crypto/Blockchain proponents and governments with central banks. Governments will continue to pressure the industry until everything is completely centralized (dependent on centralized exchanges, and other services). After all, it's no secret that today's crypto land is largely dominated by centralized exchanges. It would be surprising to see centralized mixers enforcing KYC/AML sometime in the future due to government's pressure within the mainstream world. If that happens, I believe that people will be more inclined to using non-custodial mixers than anything else. The recent situation of arresting a mixer operator, is only the beginning. Just my thoughts