They weren't at war, perhaps not, but their official diplomatic relations are a relic of the past. Neither Barca nor D'Hara could have expelled Terran without going through a war period with each other, and both realms had other priorities. That does not mean they were friendly, however, or that they planned on a long period of good neighboring.
If there was no real danger of being destroyed by D'Hara, then this cannot meaningfully count as a surrender.
That exception was written into the realm merger rule to permit a realm that has clearly lost an ongoing war to surrender before the foe that has defeated them, not to allow realms that aren't on the best of terms to have one give all its regions to the other just so the smaller can avoid being destroyed by a foe they dislike more.
