A 2d minefield can work depending on the need and angle, in example, a "curtain" of mines between a wormhole and the general space beyond it is a perfectly logical use of mines, though obviously it would still be much more effective if it had depth, if the goal is simply to force the enemy to move slower and maneuver themselves into disadvantageous formations/positions, rather than actually deal direct damage, perfectly reasonable.

That is not what is done here, but if better writers existed, even similar plot points can be made better.