Nobodies defensive contributions at 1b is disregarded. It’s just players who play at 1b start with a defensive handicap, since if they were any better they would play literally any other position.
Theoretically, if a player at 1b were so good at 1b that they completely overcame that handicap, they would be moved to another position such as third base, because it easier to find a player to play 1b than 3b, and the player has shown the ability to play 3b.
This is why we let Vlad fail at 3b for a whole year - because if he could play an average 3b, then we could just keep signing Brandon Belts to cheap one year deals instead of having to keep trading prospects or spending 20M+ a year in free agency to acquire Matt Chapmans.
It’s an economics thing.