My question is if it is the code they are using, not just the method =/
It is indeed possible to implement the Java equivalent version of the linked JavaScript, but I don’t know if it would produce the same citation keys as Papers 2/3 without knowing more about their implementation.
The issue is that a title containing {\o}, might be interpreted as,
o{\o}ø(and its different unicode variants)oe(unlikely)
which would generate different citation keys. See GitHub - retorquere/universal-citekey-js: Javascript implementation of universal cite key for some additional issues.
They are no longer using this method? Perhaps [Papers3CitationKey] would be better in that case.