Exactly. They were saying on the telecast after he botched a couple of plays against the Cubs that Miller was out working with the coaches on footwork and scooping throws. That doesn’t sound like something you do with a seasoned, polished 1B if you think his misplays are just one of those things that happen to everyone.
I don’t know how some of those advanced defensive statistics are derived, but maybe they focus more on range and fielding ground balls than the ability to come up with imperfect throws.
First base is usually seen as a position where you put a prolific hitter. Even if Miller were a gold glove fielder, the team would be getting hurt by his offense. And he is not a gold glove fielder.