It's already ridiculous that they spent several years blocking this request while it went through court. If the plaintiffs spoke to pretty much anyone involved in maintaining the system, or with any of their internal infosec people, they would know that there's no real security risk to releasing this information.
They've already spent orders of magnitude more time and money litigating the issue than it would take to just release the information in the first place, so this is clearly not a cost or resourcing issue.
They don't want to release it because they'd prefer it's secret, because secrecy makes it harder for the public to hold them accountable. That's all.
The precedent set here will let data journalists (like Matt) setup effectively automated FOIA workflows on _any_ database they can get the name of for a FOIA request. So even if _this_ db isn't dodgy it enables any of them that are to be found quickly.
Or even less cynically, its just going to cost a ton of resources to respond to all those automated FOIA requests.