Doesn’t matter. It is a valid pick, because I can change who I am guarding, or else a team can prevent any player from calling a pick by putting two players within 3 meters of him.
To say otherwise would indicate that offense gets to decide who a defender is defending (when in fact a defender may simply be guarding space that offense foolishly put multiple people in), and therefore that there is never any such thing as a valid pick call without offensive agreement. I recognize that we are trying to come to agreement anyway, but there is no way to structure the rules that allows any clarity at all for reaching agreement if the original situation can not be called as a pick.