I interpret the plural 'campaigns' as meaning Trump's GOP primary campaign and the general election campaign. It would not make sense for JS to recuse himself from investigating the Hildebeast's campaigns.
I'm with you on that, for the same reason. He shouldn't be trusted to investigate the campaign that he was invested in.
The opponent's campaign is protected against shenanigans by due process, presumption of innocence, burden of production, and burden of proof - as well as prosecutorial discretion.