"There might some really good devs who would do it just for the challenge"
There might. There was that ISEE-3 Reboot Project last year to reconnect to the International Sun-Earth Explorer 3 probe.
Were I a manager, my concern would be to have the in-house competence to be able to review those fixes. If it costs less to hire someone to make the fixes than to be able to review external contributions for the fixes, and/or if there's less uncertainty in being able to get a result in the right time frame, then I know which way I would go.
Failure to connect to ISEE-3 doesn't have the negative risk impact of making a mistake with Voyager.
Anyway, in this case it's more than just the code. The new position requires "combing through secondary documents and correspondence", and presumably that means access to the primary literature, in an archive with unscanned materials. And while some retired engineers may have no problems with others 'picking their brains directly', it would be better to coordinate those questions instead of having a dozen different people ask the same questions.