He is saying exactly that. Where as ground units have limited options in retreat and avoidence I.E. you can attack from the front and flank from the rear, left or right. In a space simulation, there is no wall to put the enemy up against. You attack from the front, and flank (not flank, would also be attack, but we'll call it flank for you) from literally any side, and they will fly through your lines, up, down, left, right. Short of making a litteral blockade of ships you can't impose flanking tactics in space. The one suprise that you can give the enemy, that would be the equivilent to a flank, is some sort of hidden reinforcements. An ambush. Which is a common air tactic (flying high altitude "In the sun"). Since there is no way to guarantee rear advantage with any one move, it's not a tactic, just an Idea, and the idea is you want to be behind the other guy.