He's right. Of course, it's to be expected that some reporters may want, seek or accept jobs with a particular campaign. So let's have some transparency.
It's only fair for the public to be informed about which journalists are seeking or being considered for jobs with the campaigns. It's a piece of information that people need to be able to take into account when they are consuming those journalists' work product.
It doesn't undermine anyone's "journalistic credibility" for them to disclose an interest in joining a campaign. In my view, it enhances it -- because they have enough respect for their public to be straightforward, and enough confidence in their own fairness that they're willing to allow their work to be scrutinized in light of that information.