Different states have different appellate rules - some places for example allow appeals only after a trial court’s case is concluded. Some permit them of any order. It sounds like Georgia is kind of in between the two, requiring the judge’s permission to appeal as the case is in progress.
Also I can’t speak to Georgia’s standards on appeal, but in New York an appeal like this, which doesn’t go to the merits of the case, is reviewed on appeal only for abuse of discretion - a very difficult bar to meet. Under such a standard an appeals court can believe the trial court to be wrong and still affirm the decision.
In McAfee’s other recent ruling, where he struck down 6 of the charges against Trump, in that decision, he left the prosecutors two options, either to re-write them in a manner more acceptable to the judge, OR that he would be willing to approve their plea to the appeals court. That is something that Trump’s team should be able to cite in their own request for appeal, to show that since he has already stated he would be willing to allow the prosecution to appeal before the end of the trial, he should allow the defense to as well, albeit on a different matter.