> It'd take effort for a mixer to log things - it's not something that happens by itself. There's no reason to think the implementor would do that, unless the mixer is already compromised.

This is true in theory but the history of software is littered with obvious counterexamples where theoretically secure software was compromised by admins forgetting to disable debugging / logging, insecure temporary or backup files, etc. When you use a mixer you're trusting the admins to get all of that right even before you talk about malice.

I'm not saying it's trivial, merely that the bitcoin infrastructure was obviously not designed to counter state-level threats. The perfect identification of all past transactions is just too easy to get wrong and and a prosecutor only needs to show a convincing case, not absolute mathematical proof – I wouldn't bet against the simple timing of transactions being enough in many cases