Had he told the jury "API's are not copyrightable", and the jury found Google guilty, and then he ruled later "API's are really not copyrightable" then he would have set aside the jury verdict of guilty.
Then, had his ruling of "API's are not copyrightable" been reversed, the jury guilty verdict would have been reinstated.
Going this route, the jury's decision is independent of the finding of "can API's be protected by copyright" because the jury found no guilt while assuming that API's could be protected. So even if the judge's decision of "no copyright on API's" is reversed, the jury's verdict of not-guilty remains, because it was made by assuming "API's can be protected by copyright".
Then, had his ruling of "API's are not copyrightable" been reversed, the jury guilty verdict would have been reinstated.
Going this route, the jury's decision is independent of the finding of "can API's be protected by copyright" because the jury found no guilt while assuming that API's could be protected. So even if the judge's decision of "no copyright on API's" is reversed, the jury's verdict of not-guilty remains, because it was made by assuming "API's can be protected by copyright".