It's nonsense. What could they offer him? seriously, would Rob Lucci forgive Spandam after what he did? He would have asked for his head in a platter before re-joining the WG. A promotion? weak shit.
I could understand them being used as characters later in the story, after all with their power level, they just needed a good navigator and they could've dealt with most of the new world bullshit if they wanted.
Also i've been re-watching water 7, and Kuzan is really a monster. He's just as terrible as Akainu, and people tend to forget. The last talk he has with Robin cements him as another psychopath, considering what he did to Saul, even knowing it was wrong.