They are not defending money laundering, they are defending coin mixing of legitimately obtained money, because many people want to outlaw coin mixing outright.
They don't think they're defending money laundering because they don't realize that what they just "invented" is money laundering.
The analogy doesn't make sense, since the difference between the mixer and the jar is not the place that the "money" is mixed, but more like the difference between sex and rape. It doesn't matter in which room the act took place.
It's missing the point, just like saying you thought you were relying on free speech when you signed a contract and then didn't hold up your part of the deal.
Sure. And I would have no problem with that. Probably neither would the law.
But if you want to use a mixer to increase your privacy, then you are sophisticated enough to realize that you're an active participant in other people's moneylaundering.
The analogy with the jar would be more complete if you know that one of the people putting money into the jar happens to put in fresh sequentially numbered bills. A reasonable person can easily deduce that they are therefore becoming part of a criminal act.
If a friend of yours asked to mix their pile of money with yours, because theirs is all sequentially numbered, would you do it? Would you admit that you're probably helping them commit a crime, if you do?
Legal commentators on SBF have said that part of the reason his defense was weak is that even if he didn't know about the fraud, it was so blatantly obvious that fraud was happening and he continued taking further actions enabling the fraud, that would make him legally liable for the fraud even if that story is true.
They are not defending money laundering, they are defending coin mixing of legitimately obtained money, because many people want to outlaw coin mixing outright.