A drain AND flat aren't two things that go together....
So if there's a drain involved - they would have sloped the floor to the drain.
I do agree with Jer though! If the contract says flat - and you want flat - it will drive you nuts every day that it's not.
We jackhammered 50 cubic yards out of our backyard around the pool because it started to crack and the cracks drove me nuts! Only one way to fix that!
You see all the dirt area - and where the new pavers are? ALL of that got jackhammered out and it was laced with #4 rebar! Then we replaced it with the pavers. The jackhammering went on for about 2 weeks!
BUT I was happy we bit the bullet!