You can't have 2500 people in the same area in this game. You can't even have 100, because the game will spawn multiple instances of the zone and segregate you all. The entire game is about segregating the playerbase into their own little rooms so they can feel like their decisions matter, but they never do.
Some instancing is done, yes, but not at the 100 level. I've been on instances of the Fleet and others where there were 200+ people in one instance. And I'm not sure how that would interact if it was 200 Republics and 200 Imperials at the Nar Shaddaa Promenade where both groups can mingle together.
But the presence of instancing doesn't change it from being an MMO, especially since you're in the same instance chat-wise if not character-wise, and can switch instances at will.