This is a state case. Referral for criminal charges goes to a district attorney in Northern California. Trump's DOJ could try to lean on California but no one in California has any taste for Trump and his people.
You seem to have made the mistake of thinking that a news article saying “a judge in northern California" means “a State of California judge in the northern part of that state" rather than “a federal judge in the Northern California District Court”.
But most of the Central Valley and the inland Northern California counties are much more Republican than Orange.
(I mean, sure, by population they mostly do, and are overwhelmingly Democratic, but if you are going to look for a county that goes against the partisan trend of the state, staying in the urban coastal enclaves and picking Orange is actually a fairly weak example.)