Which means there are really three red flags: internal tools deployed to the public internet, internal tools without any user access control, and allowing passwords to hit the logging infrastructure.
This is the most ridiculous of the three in my opinion. When logging to elasticsearch you programmatically have to say what data to put in the document before its indexed (and thus viewable via Kibana). This means that the developer actually put the password as a parameter in the document object.
This is a modern issue with the way laws look at subcontracting work as a tool that allows companies to isolate themselves from liability which needs to change, if your company (Elsevier) sub-contracts it should be your company on the line - with an allowance to pursue legal action against the sub-contractors that will be examined in depth during that proceeding.
These sorts of situations have too often resolved in "Well, we can't tell if it was the parent or sub-contractor that was responsible, I guess we need to let them both off." instead hit the parent with the full force of the law as a negligent reseller and if they were unaware of these situations they can recoup their costs by pursuing the subcontractor on their own dime.
To that I say meh: Could just as well be that they decided to log all Http request to troubleshoot some issue without realizing the security implications. The article even says it was a rolling list of passwords which further indicates loggimg since you probably don't store trace logs forever.