The judge presiding over the case is Aileen Cannon, who has previously ruled in Trump’s favour on this case by putting an injunction on the investigation. Her ruling was baseless and the appeals court ripped her to shreds over it.
She could throw out the case or sentence Trump to probation (even if the “suggested” sentence is 15 years) or do all kinds of things that an appeals court could do little about. For an appeals court to overturn a ruling they have to find a procedural or legal error. A judge can do a lot of questionable stuff without making any legal errors.