The difference here is that a cryptocurrency mixer serves exactly one purpose: to be used to provide a financial service. You're not going to be able to plausibly argue that you didn't know what it was going to be financial service, and you're probably not going to be able to plausibly argue that you didn't know it was going to violate applicable financial regulations.

When you're working on other privacy/security technologies, they have substantial enough other uses that you are plausibly able to argue that you were ignorant of its use in illegal steps.