I have no idea exactly how this site is built. However any system I have worked on could link logfiles which would link an IP address with anything downloaded from the site. That could be chained into logfiles about user logins to know who downloaded what. That would be enough for the police to get a warrant based on suspicion of possession of CSA if they so wished. Whether they would do that to target specific MN accounts would depend on how thick your tinfoil hat is.
Not being a lawyer my knowledge of the intricacies of the law are limited. However the offence occurs regardless of intent, so you really have to rely on prosecutorial discretion. Whether that alarms or comforts you will be a personal choice. Worth noting that MN would be unable to confirm or deny this under the law.
Regardless that action is now out of MNs hands. Presumably police computer forensic teams will have obtained the relevant logfiles and be trawling as we speak.
Things could have been worse. MN could have a system that emailed thread updates to subscribers which would have been incredibly bad for anyone subscribed.