Glad you asked !
The realistic space opera line is self-contradicting, really. Star Wars for example is anything but realistic on pretty much every level (still awesome). We’re two developers and we’re both engineers, we love hard sci-fi, so we wanted to create a world based on actual physical constraints, and still keep the awesome of Star Wars or Battlestar Galactica. We simulate heat, cooling by thermal radiation, our guns fire smart shells, we have a Newtonian flight model and no sound coming from outside the ships. We stay in orbit around a single planet and we don’t have time travelling, laser-firing spaceplanes.
What we want to give the player is freedom to explore, or trade, or fight other factions, and a setting that makes it interesting. Games like X3 would let you build gigantic fleets of warships, but there wasn’t really anything to do with it. So we’re trying to build a world that gives you a lot to discover, doesn’t try to destroy you when you’re playing nice, gives you a challenge and a reward for fighting. We want the player to organize large battles on his own because he’s following a plan, like in a Total War game, but we don’t want to make that a vital necessity (like in a Total War game).
TL;DR: What we really mean in realistic space opera is that it’s a mashup of different game styles.