I completely understand all of this. But AFTER a player is already signed, it really no longer matters.
Last year, Soto was -8 runs prevented, and -9 OAA. So defensively, he cost his team 8 runs, and 9 outs. By simply getting him out of the OF and having him DH increases his field value to the team. I believe that positional adjustment only really works if a player is a positive defensive player. Taking a positive defensive player and pulling him off the field to be the DH, hurts the field value for the team. If a guy is costing them runs and outs while being out there, by removing him and having him just hit, obviously helps the field value for the team.
There's nothing value wise that's negative by taking a great hitter, who loses value because of his defensive, and not having him play defensive anymore. All his negative value is now removed from the equation. He's now more valuable. He has more value to the team by DH'ing, and not playing OF.