It ‘assumes’ no such thing.
If a district is drawn in such a way that only one party has a chance of winning, then no one has a choice.
Your vote literally doesn’t matter… no matter who you might prefer to vote for.
If they ‘stack the deck’, they tend to do so with at least a modicum of fairness. If they didn’t, then no Democratic district would ever flip.
Not ‘if’, but when, since they can absolutely be relied on to rig the game in their favor every way they can; and it absolutely is about race.
Know your dog whistles…