I don't think there's any causation from the USA sanctions to the arrest, possibly even vice versa, with some investigation before this arrest leading to some part of the sanctions.

The specific USA sanctions are not retroactive, it's not a crime to have violated them before an entity was sanctioned; and I doubt if it's a Netherlands crime to violate USA sanctions.

In any case, the article seems quite clear that he was arrested for facilitating money laundering, not for a sanctions-related offense, the sentence about sanctions was just extra information for context. And for this part, the big issue is intent - if it turns out that he developed this mixer with the intent to facilitate hiding illegal transactions, that would be facilitating money laundering. And looking at some comments here I wouldn't be that surprised that he might have explicitly admitted it in writing.

They were waiting for the reactions from the community. There's a low chance there could have been extreme and unified non-compliance, e.g. everyone depositing funds to Tornado Cash (which is still up and operating at https://cloudflare-ipfs.com/ipns/tornadocash.eth/ )