Were Terran and D'Hara at war before this?
Because if not, I think that the "surrender" argument is on shaky ground.
I think it's also appropriate to point out that despite their being Magistrates, Vellos and Chénier are both directly involved in this case, and thus can only speak as well-informed players.
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.