CathInfo.com

The way it's worded, it sounds like the hosting site needs to be served a DMCA notice first, then any failure to respond would be a violation. As it stands right now, cathinfo is not hosting anything, just a thread that contains a link to a third party, not the actual copyrighted work. 

"However, even when webmasters and hosts have not participated directly in wrongdoing, they might be found to contribute to copyright violations through negligence if they do not take proper steps to curb copyright violations."

How would they know it's a copyright violation without first receiving a DMCA takedown notice? What are the legal "necessary steps" to curb violations?

I am presuming the District is already in the process of instructing its legal counsel to send Matthew the DMCA letter.

If in the next three years, he does not receive a letter, he is probably home free (i.e., the statute of limitations for copyright infringement in the US is currently 3 years).

But I am having a difficult time believing, in light of their litigious history on this particular subject, they will just let the matter pass without any response at all.

They would as much as be giving the green light for all to disseminate the collection far and wide, where their past history shows a keen interest in suppressing it.

Common steps to curb violators and violations in this case might include the removal of the the links enabling the infringement.  In this case, it might be helpful to delete the links before they reach their 7-day termination point, which could show good faith against any action for negligence.

As for webmasters protecting themselves against potential infringement suits, see here: https://blogs.lawyers.com/attorney/copyrights/webmaster-liability-for-copyright-infringement-10443/

I am not an attorney, and this is not legal advice.