As long as they report the issue upstream in that library I agree with you.

“More specifically, the underlying cause was a bug in a so-called third-party library. The software component is used to communicate between Nordnet's services and the database that stores information about which customers are logged in.” As an IT I could be already using that library at work:

IMO we should keep pushing Nordnet to publish a full technical post-mortem as it would be a good thing for both the IT industry and Nordnet regain their trust back.

That guess doesn’t sound right as only customers logged in during the incident were affected (ie potentially exposed to other accounts). So that points to a cache issue of some sort.